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A Proposed Hybrid Vehicle Architecture Based on Direct Electrical Coupling Between Matched Machines

A Brief Introduction for Non-Specialist Readers

Most hybrid vehicles use one of two main approaches. In a conventional parallel hybrid, the petrol engine can drive the wheels through a mechanical transmission. In a series hybrid or range-extended electric vehicle, the engine generates electricity, which passes through power electronics before reaching the traction motor.

The proposed Direct Electric Drive EV system (DEDEV) takes a different approach. A petrol engine drives a generator, which supplies a rear electric traction motor through a direct three-phase electrical connection during suitable highway cruising conditions. This operating mode is intended to avoid a conventional traction inverter in the rear propulsion power path.

The vehicle also has an independent front electric drive powered by a battery. This front drive provides electric launch, additional acceleration, and regenerative braking.

The generator and rear traction motor are designed as closely matched, or ideally identical, electrical machines. Their speeds and electrical waveforms are synchronised before the direct connection is established.

The objective is to create a hybrid system with an electric-drive character, no conventional clutch or multi-speed gearbox in the proposed propulsion path, and reduced reliance on continuously operating rear-drive power electronics during steady cruising.

DEDEV is a proposed architecture, not a proven production technology. Its efficiency, thermal benefits, safety, durability, and cost must be established through engineering analysis and experimental validation.

Part I — Technical Description for Engineers

1. Executive Summary

The Direct Electric Drive EV system (DEDEV) is a proposed hybrid vehicle architecture combining two electrically independent propulsion paths:

1. A front battery-electric drive, incorporating a traction inverter and a front electric motor.

2. A rear direct-electric drive, comprising a petrol engine, generator, switching and protection system, and rear traction motor.

During suitable highway operation, the generator is electrically connected to the rear traction motor through a three-phase contactor assembly and a protected high-voltage cable. In this direct operating mode, the rear traction power is transferred without passing through a conventional traction inverter in series with the rear motor.

The front electric drive maintains vehicle propulsion during engagement and disengagement, provides additional torque when required, and recovers braking energy within the limits of battery acceptance and vehicle stability.

The central design hypothesis is that closely matched generator and motor machines can simplify electrical coupling and reduce conversion losses during steady cruising. This potential benefit must be assessed against the limitations of direct synchronous operation, the additional switching and control hardware, the engine operating map, and the performance of modern series-hybrid power electronics.

DEDEV is proposed as an independent hybrid architecture for further research. It is not claimed to be inherently superior to parallel hybrids, conventional series hybrids, or range-extended electric vehicles.

2. Design Objectives

The proposed architecture has six primary objectives:

• Enable direct electrical power transfer from the generator to the rear traction motor under suitable operating conditions.

• Reduce the need for continuous switching and power conversion in the rear traction power path during direct cruising.

• Retain battery-electric propulsion through the front axle.

• Provide front-axle torque assistance during acceleration, overtaking, and uphill driving.

• Recover braking energy through the front electric drive, subject to battery and stability constraints.

• Avoid a conventional clutch and multi-speed gearbox in the proposed propulsion arrangement.

A secondary objective is to investigate whether reduced rear-drive inverter activity can improve thermal operating conditions during extended highway driving.

No specific fuel-saving percentage, cooling reduction, or efficiency advantage is assumed to have been demonstrated.

3. Proposed Vehicle Configuration

3.1 Front axle: independent battery-electric drive

The front propulsion system comprises:

• A high-voltage traction battery.

• A bidirectional traction inverter.

• A front electric traction motor.

• A fixed-ratio final drive.

• Battery, motor, inverter, and vehicle-level thermal management.

• Supervisory controls for propulsion, regenerative braking, and torque coordination.

The front axle is responsible for electric launch, low-speed driving, additional acceleration, and regenerative braking. It can also provide torque while the rear propulsion path is being connected or disconnected.

The front axle is electrically independent of the rear generator-to-motor propulsion path. The proposed architecture does not require a direct electrical connection between the front battery traction circuit and the rear traction motor during direct cruising.

3.2 Rear axle: direct-electric propulsion

The rear propulsion system comprises:

• A naturally aspirated 1.5-litre petrol engine using an Atkinson-cycle operating strategy, as an initial candidate.

• A fixed-ratio engine-to-generator drive.

• A synchronous generator.

• A generator-side bidirectional power converter for engine starting, series operation, charging, synchronisation, and applicable current-control functions.

• A three-phase switching assembly with redundant contactors and monitoring.

• A shielded and insulated high-voltage three-phase cable.

• A rear electric traction motor.

• A fixed-ratio rear final drive.

The engine and generator are proposed to be installed at the front of the vehicle, close to the radiator and the main engine cooling system. The three-phase cable runs to the rear traction motor through a dedicated protected channel beneath the vehicle.

The cable installation must account for electrical insulation, impact protection, abrasion, water and salt ingress, heat exposure, electromagnetic compatibility, mechanical retention, serviceability, and applicable vehicle safety requirements.

The rear motor remains mechanically connected to the rear wheels. Consequently, it rotates whenever the vehicle moves, even when it is not receiving propulsion power from the generator.

4. The Twin-Machine Matching Principle

4.1 Design hypothesis

The generator and rear traction motor are proposed to be electrical and mechanical twins, ideally sharing the following characteristics:

• The same number of pole pairs.

• Closely matched winding geometry and electrical characteristics.

• Closely matched permanent-magnet flux linkage, if permanent-magnet machines are used.

• Compatible back-electromotive-force characteristics.

• Compatible voltage, current, speed, torque, and thermal ratings.

• Compatible phase sequence and electrical reference conventions.

The purpose of this matching is to simplify synchronisation and direct electrical coupling.

For a synchronous electrical machine, the electrical frequency is related to mechanical speed by:

[
f=\frac{p n}{60}
]

where:

• (f) is electrical frequency in hertz.

• (p) is the number of pole pairs.

• (n) is mechanical speed in revolutions per minute.

If the generator and motor have the same number of pole pairs and rotate at the same mechanical speed, their electrical frequencies will be equal.

If their voltage constants and winding characteristics are also matched, their open-circuit voltage waveforms may be closely matched at corresponding speeds.

However, identical machines do not automatically synchronise simply because they have the same design. Their actual speeds, electrical angles, terminal voltages, phase sequence, and operating states must still be measured and verified.

4.2 Relationship between vehicle speed and machine speed

The rear motor is permanently coupled to the rear wheels through a fixed final-drive ratio. Its speed is therefore determined by vehicle speed, wheel radius, and final-drive ratio.

The engine speed is determined by the generator speed and the fixed engine-to-generator ratio.

Consequently, the engine speed is locked to road speed during direct operation. Engine speed cannot be independently selected to follow the most efficient operating point of the engine.

This is a central design trade-off: the fixed mechanical ratios must be selected to provide a workable compromise between engine efficiency, generator and motor speed, high-speed power capability, and electrical frequency.

4.3 Why matching helps, but does not eliminate control requirements

The twin-machine principle may simplify the process of achieving compatible electrical conditions before connection. It does not remove the need to verify:

• Frequency difference.

• Phase-angle difference.

• Voltage-magnitude difference.

• Current and torque transients during connection.

• Synchronising stability under load.

• Safe current management during disconnection.

• Maximum-speed back-EMF and insulation requirements.

The design must establish experimentally whether the resulting pull-in torque, synchronising margin, transient current, and dynamic response remain within acceptable limits.

5. Operating Modes

The system may also support battery charging through series operation or controlled load shifting. These modes must be defined separately from the direct propulsion mode because they involve different electrical power paths and conversion losses.

6. Engagement Strategy at Approximately 70 km/h

A proposed transition to direct operation occurs at approximately 70 km/h, provided all enabling conditions are satisfied.

This speed is a preliminary calibration target, not an absolute requirement.

6.1 Stage 1 — Front-axle torque support

The front electric motor maintains the requested vehicle speed and longitudinal acceleration during the synchronisation process.

Because the rear traction motor is mechanically coupled to the wheels, it is already rotating at the speed corresponding to vehicle speed. It is not necessary to accelerate the rear motor from rest.

The generator-to-motor contactors remain open during this stage.

6.2 Stage 2 — Engine and generator speed matching

The supervisory controller checks vehicle speed, sensor validity, traction conditions, ABS/ESC status, battery capability, contactor status, and generator-converter readiness.

The engine is then controlled so that the generator reaches the electrical speed corresponding to the rear traction motor.

The engine and generator speed relationship is determined by the fixed mechanical ratio.

6.3 Stage 3 — Electrical synchronisation

The controller measures the voltage waveforms at the generator and rear motor terminals, or uses suitable rotor-position and electrical-angle measurements.

Before connection, the controller verifies that:

• The frequency difference is within the validated limit.

• The phase-angle difference is within the validated limit.

• The voltage difference is within the validated limit.

• The matching conditions remain stable for the required validation interval.

• No safety interlock prohibits engagement.

The contactor closing command must account for measured contactor closing time and its variation. Successful physical closure must be confirmed by independent monitoring.

6.4 Stage 4 — Torque transfer

After successful connection and confirmation that the electrical and mechanical response is acceptable, engine torque is increased progressively while front-motor torque is reduced.

The objective is to maintain the requested total vehicle torque throughout the transition.

If the engagement process does not meet the required limits, the system remains in or returns to a safe alternative operating mode rather than forcing the connection.

The engagement strategy must be validated for transient current, torque disturbance, torsional oscillation, and synchronising stability.

7. Redundant Contactor Arrangement

7.1 Proposed configuration

The direct three-phase link is proposed to use two independently controlled contactors connected in series, designated:

• K1 — Primary contactor

• K2 — Backup contactor

During normal direct operation, both contactors are closed to carry the traction current.

If K1 is commanded to open but monitoring detects that it remains closed or has failed to interrupt the circuit, K2 is commanded to open to isolate the direct link.

This arrangement provides a second isolation device if the primary contactor fails in the closed state.

7.2 Fault detection and response

The controller must distinguish between a contactor that is intentionally closed during direct operation and one that has failed to open after an opening command.

Fault detection may use a combination of:

• Independent contact-position feedback.

• Voltage measurements across each contactor.

• Current measurements in the direct link.

• Command-versus-response timing checks.

• Electrical continuity diagnostics where applicable.

If K1 is confirmed to have failed to open, K2 should receive an independent opening command without unnecessary delay.

The backup contactor should have an independent control path and sufficient documented interruption capability for the credible fault conditions it may encounter.

7.3 Important limitation

Opening K2 is not automatically safe merely because it is the backup device. If the primary contactor is welded and current is still flowing, K2 may be required to interrupt that current.

The design must therefore establish:

• Whether the generator-side converter can reliably reduce or divert current in the relevant fault state.

• What happens if that converter is unavailable.

• The current and voltage K2 must interrupt under the worst credible fault.

• The resulting arc energy, transient overvoltage, and insulation stress.

• Whether the system can maintain vehicle stability while the rear drive is isolated.

Both contactors being closed during normal direct operation does not mean both should be closed in the system's safe, de-energised state. When direct operation is not active, the intended safe state is for the direct link to be isolated.

The contactor arrangement should be validated through fault-injection testing, including simulated welded contacts and loss of control power.

8. Disconnection Strategy

Under normal conditions, the preferred sequence is:

1. The front axle prepares to maintain the requested vehicle torque.

2. Engine torque is reduced as appropriate.

3. The generator-side converter manages current in accordance with its validated current-control strategy.

4. The primary contactor is commanded to open.

5. Independent feedback verifies that the primary contactor has opened.

6. If the primary contactor fails to open, the backup contactor is commanded to open.

7. The controller verifies isolation and transitions to the appropriate alternative mode.

The current-diversion strategy must be designed around the actual electrical topology. Reducing generator current alone does not necessarily prove that current through the contactor has reached zero, because current in the motor branch and the converter branch may differ.

The contactors must therefore be rated for their credible interruption duties, including defined abnormal conditions. The design must not depend solely on an assumed zero-current opening unless that condition can be demonstrated and monitored.

A contactor stuck closed, a loss of synchronisation, and a loss of converter control must each have a separately validated response.

9. Preliminary Vehicle Assumptions

The following values provide an initial calculation basis. They are not final vehicle specifications.

The engine's rated power, torque curve, continuous operating capability, fuel-consumption map, and thermal limits must be obtained from an actual candidate engine before component sizing is finalised.

10. Engine-Speed Selection and Gear-Ratio Implications

A preliminary target is approximately 1,750 rpm at 70 km/h.

Assuming a fixed relationship between vehicle speed and engine speed, the corresponding engine speeds would be:

These values are conditional on the selected fixed ratios.

With a 9:1 rear final drive and a 0.33 m dynamic wheel radius, the rear traction motor would turn at approximately 5,060 rpm at 70 km/h. To achieve an engine speed of 1,750 rpm at that road speed, the generator-to-engine speed ratio would be approximately 2.9:1, subject to the exact definition of the ratio and real drivetrain dimensions.

This differs from a ratio of 2.5:1, which would produce an engine speed of roughly 2,025 rpm under the same assumptions.

The final ratio must therefore be recalculated consistently with the selected motor speed, engine operating map, generator limits, and maximum vehicle speed.

The preferred engine speed cannot be selected from rpm alone. It must be assessed against the engine's brake-specific fuel consumption (BSFC) map and the continuous power required at each road speed.

11. Preliminary Road-Load Power Estimates

Using the assumptions in Section 9, the following are first-order estimates of steady road-load power in still air.

The 87.6% path efficiency is an illustrative product of preliminary component-efficiency assumptions. It is not a measured system efficiency and should not be treated as constant across speed, torque, and temperature.

The 160 km/h headwind case requires approximately 53.8 kW at the engine shaft under these assumptions. A design target based only on the engine's advertised peak power is insufficient: the engine must supply the required continuous power at approximately 4,000 rpm, with appropriate thermal and operating margin.

Gradeability, vehicle payload, tyre characteristics, accessory loads, ambient temperature, and transient acceleration must also be included in final sizing.

12. Preliminary Rear-Machine Ratings

For the next stage of engineering evaluation, the following may be used as candidate sizing targets, rather than validated specifications:

• Rear traction motor continuous rating: approximately 60 kW.

• Generator continuous rating: matched to the motor and the engine's continuous power capability.

• Short-duration peak rating: approximately 75–80 kW, subject to machine speed, current, voltage, thermal limits, and test results.

• Maximum mechanical speed: sufficient for the target vehicle speed with an appropriate overspeed margin.

• Electrical insulation: rated for the maximum credible open-circuit voltage, including transient and overspeed conditions.

The generator and rear motor must be matched in continuous rating as well as electrical characteristics. A nominal 60 kW motor cannot be assumed to operate continuously at that power unless its cooling and speed-dependent limits permit it.

If the engine cannot supply the required power at 4,000 rpm, increasing the motor and generator ratings will not solve the engine power limitation. The engine selection, fixed ratio, target performance, and expected front-axle contribution must be considered together.

13. Electrical Frequency and High-Speed Operation

For a machine with four pole pairs, the electrical frequency is:

[
f=\frac{p n}{60}
]

Using the approximate rear motor speeds derived from the 9:1 final drive:

These values are approximate and depend on the final-drive ratio, dynamic wheel radius, and pole-pair count.

The high electrical frequency requires careful evaluation of:

• Stator iron losses and winding AC losses.

• Rotor and permanent-magnet eddy-current losses, where applicable.

• Insulation stress and switching or transient voltage.

• Bearing, rotor, and mechanical overspeed limits.

• Back-EMF at maximum speed.

• Thermal behaviour under sustained highway operation.

A rotating permanent-magnet traction motor may generate significant terminal voltage even when disconnected from the generator. This condition must be included in insulation, sensing, isolation, and fault-response design.

14. Front Electric Drive and Battery

An initial battery capacity of approximately 11 kWh may be used for system-level studies, with a nominal voltage in the 400 V class.

Candidate front-drive targets may include:

• Front motor peak power of approximately 90 kW.

• Front motor continuous power of approximately 50 kW.

• A bidirectional inverter sized for the intended motoring and regenerative-braking duties.

• Liquid cooling and battery preconditioning as required by the selected cells.

• A battery management system that limits power according to state of charge, temperature, voltage, and cell ageing.

These figures remain preliminary.

A proposed regenerative charging power of 65 kW into an 11 kWh battery corresponds to approximately 6C relative to nominal capacity. Such a rate must not be assumed acceptable for an unspecified battery chemistry. Cell-level charge acceptance, temperature limits, ageing, and pulse duration must be verified from the selected manufacturer's data.

Regenerative braking should be blended with friction braking whenever battery acceptance, traction, temperature, or inverter limits prevent the requested braking torque from being recovered electrically.

15. Energy Management

The supervisory controller should coordinate engine operation, direct-link availability, battery state of charge, front-axle torque, and regenerative braking.

A preliminary strategy may include:

• Preserving battery capacity for acceleration assistance and regenerative-braking headroom.

• Entering direct mode only when its synchronisation and safety conditions are satisfied.

• Using the front axle to maintain requested vehicle torque during transitions.

• Giving priority to regenerative braking when it is safe and the battery can accept the energy.

• Using controlled engine-driven charging when required by the energy-management strategy.

• Avoiding unnecessary battery charging during periods when the stored energy is unlikely to provide a useful operational benefit.

Charging the battery by applying additional generator load necessarily increases the mechanical power required from the engine. It should therefore be evaluated as an energy-management choice, not as a source of free energy.

16. Thermal Management and Efficiency Assessment

The principal thermal hypothesis is that direct generator-to-motor operation can avoid some losses associated with conventional rear traction power conversion.

However, the generator, rear motor, petrol engine, front inverter, front motor, and battery still generate heat. The generator-side converter also remains part of the system for functions required outside direct operation.

Consequently, the thermal benefit must be assessed separately in terms of:

1. Losses avoided in the rear propulsion power path.

2. Electrical energy consumed by cooling pumps and fans.

3. Total heat rejected by the vehicle.

4. Cooling-system capacity required under maximum continuous load and hot ambient conditions.

No specific percentage reduction in total cooling demand or fuel consumption should be claimed until validated measurements are available.

A meaningful comparison should use the same vehicle, road-load conditions, drive cycle, ambient temperature, payload, battery state-of-charge strategy, and performance requirements.

The reference systems should include at least a modern series-hybrid or range-extended architecture and a suitable parallel-hybrid architecture.

17. Vehicle Stability and Torque Coordination

During direct operation, rear-wheel torque is governed primarily by the engine, generator, and coupled electrical-machine dynamics. The rear axle does not have the same freely commanded, high-bandwidth torque control available from a conventional inverter-driven traction motor.

The controller must therefore determine when direct operation is permitted and when the link should be disconnected.

Conditions requiring particular attention include:

• Active ABS, ESC, or traction-control intervention.

• Low-friction surfaces.

• Significant cornering or abrupt changes in vehicle dynamics.

• Hard braking.

• Rapid changes in driver torque demand.

• Loss of synchronisation or abnormal current.

• Sensor, communication, or converter faults.

The front axle may compensate for changes in rear torque, but compensation is limited by front-motor capability, battery power, tyre grip, and control latency.

The safety analysis must consider the interval between detecting a fault, commanding disconnection, and physically interrupting the electrical link. Opening a contactor is not an instantaneous torque-control action.

18. Functional Safety and Fault Management

The design should follow a systematic vehicle-safety process incorporating failure-mode analysis, fault-tree analysis, fault injection, and the applicable functional-safety requirements.

Important fault cases include:

The two-contactor arrangement improves isolation redundancy only if the contactors, drivers, monitoring, wiring, and control logic do not share a common failure that defeats both isolation paths.

The backup contactor must be assessed for interruption capability under the actual electrical fault conditions, not merely for its ability to open mechanically.

19. Validation Programme

Development should proceed in stages.

Stage 1 — Analytical and computational modelling

• Finalise the gear ratios and machine-speed relationships.

• Calculate torque, power, voltage, and frequency over the full speed range.

• Build generator and motor efficiency maps.

• Analyse synchronising stability and transient torque.

• Model the electrical behaviour of both contactors and the converter.

• Evaluate thermal performance and open-circuit voltage.

Stage 2 — Dynamometer and electrical bench testing

• Test matched generator and motor machines under controlled conditions.

• Measure voltage, phase, frequency, current, and torque during connection.

• Test normal and abnormal disconnection.

• Inject faults involving sensors, converter control, and contactor feedback.

• Verify the response to a simulated welded primary contactor.

• Confirm the backup contactor's interruption capability under representative fault conditions.

Stage 3 — Energy and thermal validation

• Measure direct-path efficiency across speed and load.

• Compare fuel consumption with suitable reference architectures.

• Measure cooling-system power consumption and component temperatures.

• Validate battery charging and regenerative-braking limits.

• Evaluate sustained high-speed operation in hot ambient conditions.

Stage 4 — Vehicle-level validation

• Test the transition between electric and direct modes.

• Evaluate torque continuity and driver-perceived smoothness.

• Test overtaking, gradients, braking, and low-friction conditions.

• Verify interaction with ABS, ESC, and traction control.

• Conduct durability testing and safety validation before any public-road deployment.

20. Conclusions

The Direct Electric Drive EV system is a proposed hybrid architecture built around a distinctive operating mode: direct electrical coupling between a generator and a rear traction motor, supported by an independent front battery-electric drive.

Its potential advantages include reduced dependence on continuous rear-drive power conversion during direct cruising, electric launch and front-axle assistance, and a propulsion layout that does not require a conventional clutch or multi-speed gearbox in the proposed drive paths.

The twin-machine concept may simplify frequency and voltage matching, while the redundant contactor arrangement may improve the ability to isolate the direct link after a primary switching failure. Neither feature eliminates the need for synchronisation control, transient analysis, fault detection, or experimental verification.

The principal unresolved questions concern synchronising stability, safe current interruption, high-frequency machine losses, engine power at the selected fixed ratio, sustained thermal performance, and the real-world efficiency and cost trade-off against existing hybrid architectures.

DEDEV should therefore be presented as a distinct proposed hybrid architecture whose benefits remain to be demonstrated, rather than as a proven improvement over existing systems.

Its engineering value will ultimately depend on whether the direct electrical link delivers measurable system-level benefits under the driving conditions for which it is intended, while meeting the required safety, durability, performance, and cost targets.

Claude AI Layout:

Direct Electric Drive EV system (DEDEV)

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DEDEV is a plug-in series-parallel hybrid architecture whose parallel mode is realised by a direct electrical link between twin machines, instead of a clutch and gearbox.

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Part A: Introduction for non-specialists

Most hybrid cars link the engine to the wheels in one of two ways. In a parallel hybrid, a clutch and gears connect the engine to the wheels. In a series hybrid or range extender, the engine only makes electricity, and all of it passes through power electronics (inverters) before it reaches the wheels.

The Direct Electric Drive EV system (DEDEV) uses a third approach:

• The petrol engine drives a generator.

• The generator and the rear electric motor are identical twins. At any given speed they make and use electricity at exactly the same frequency.

• At highway speed, a heavy-duty switch (a contactor) joins the generator to the rear motor with a cable. Power flows straight from one to the other, like a shaft made of electricity.

• There is no clutch, no multi-speed gearbox, and no inverter in the power path while cruising.

• In town, the car runs on the battery through the front motor. The front motor also recovers braking energy, and adds a boost when you overtake.

The aim is to keep the simplicity of an electric drive and cut the electronics’ workload on long, steady highway trips. DEDEV does not claim to be more efficient than a good parallel hybrid. It is an engineering trade-off with its own risks, and the safeguards in this document are part of the design.

Part B: Technical description

1. Scope and assumptions

All figures are first-order estimates and must be verified by simulation and test. The patent and trademark landscape has not been searched. The “electric shaft” idea is historically known (locomotives, ships), so DEDEV is presented as a configuration, not as a new principle.

2. Architecture

FRONT AXLE (battery system)             REAR AXLE (electric shaft)

Battery <-> Traction inverter <-> Front  Engine -> Fixed step-up gear (i ≈ 2.5)

motor (always coupled to wheels)         -> Generator (TWIN of rear motor)

                                         [Generator inverter on same terminals]

                                             |

                                         3-phase contactor(s) -> shielded cable

                                             |

                                         Rear motor (TWIN, no inverter) -> final drive

• Front: a conventional EV drive. It handles city driving, regeneration and boost.

• Rear: engine, fixed gear, generator, contactor, cable, rear motor. The rear motor has no inverter.

• Generator inverter: a permanently connected bidirectional converter on the generator terminals. It does engine cranking, series-mode charging, synchronisation, and current diversion before contactor opening.

• No clutch and no multi-speed gearbox. Fixed ratios remain: the final reduction and the engine-to-generator gear, as in current hybrids.

• Battery: medium size, plug-in capable.

• Layout: the engine and generator can stay at the front of the car. The “shaft” is a shielded three-phase cable running to the rear motor.

3. The twin-machine principle

The generator and rear motor are electrically and mechanically identical: same pole-pair count, winding, EMF constant and rating. For a synchronous machine, electrical frequency is f = p·n/60. If both machines have the same p and turn at the same speed n, they run at the same frequency and develop the same EMF amplitude. This is why the contactor can close with almost no current.

Consequences:

• The generator must turn at the rear motor’s speed: n_gen = n_motor = (wheel speed) × (final reduction).

• Engine speed is set by the fixed gear: n_engine = n_motor / i.

• In direct mode the engine speed is therefore locked to road speed. The only free variable is load, not speed.

• Operating points are chosen through the gear ratio i and the load-shifting strategy, not through the electrical drive.

• Any asymmetry between the two machines (a different p, or a different EMF constant) breaks the matching. It would need a different gear ratio or a different strategy.

4. Operating modes

5. Design equations

1. Electrical frequency: f = p·n/60. Example with 4 pole pairs: about 337 Hz at 70 km/h and about 772 Hz at 160 km/h.

2. Engine speed: n_engine = n_motor / i.

3. Link power is limited by the engine shaft power minus losses, and link torque is limited by the pull-out torque of both machines, with margin.

4. DC-link voltage must exceed the peak back-EMF at maximum speed, otherwise the generator inverter’s diodes rectify uncontrolled.

5. The open-circuit rear motor is a live source whenever it turns. Its insulation must be rated for the open-circuit voltage at overspeed plus transient margin.

6. Synchronisation sequence

1. Pre-conditions: speed in window, adequate traction, no ABS/ESC activity, sensors healthy, generator inverter armed, both contactors healthy.

2. The generator inverter brings the engine/generator to the electrical speed of the rear motor (PLL-type loop).

3. Phase match using generator angle and rear motor angle (new position sensor, or zero-crossing detection from terminal voltage). Voltage match by a small d-axis current.

4. Verify that differences are inside narrow limits and stable for several hundred milliseconds.

5. Issue the close command ahead of the match instant by the contactor’s mechanical closing time (about 20–50 ms), and verify after closing.

6. Hand over torque gradually: engine torque rises while front-motor torque falls, and the sum stays constant.

7. Both inverters return to armed standby.

Closing is done at near-zero engine torque, then loaded.

7. Disconnect sequence

1. The front inverter takes over torque while engine torque is reduced.

2. The armed generator inverter diverts current away from the contactor (contactor current = generator current − inverter current), bringing it near zero within milliseconds.

3. The contactor opens with almost no arc.

4. The engine stops, or continues to charge.

8. Armed (standby) inverter

In cruise the gate drivers are off, so there is no switching loss and no conduction loss. Gate-drive supplies, current sensors, the resolver and control remain powered (tens of watts), so current diversion can start within fractions of a millisecond. The converter is fully off the power path in terms of heat, but not in terms of connection.

Loss of “armed” status is treated as a reason to prevent or release the link.

9. Rear motor and generator sizing (160 km/h)

Road-load power (still air):

Results:

• Rear motor and generator (twins): 55 kW continuous, about 70 kW peak (≤60 s). This covers 160 km/h in 10 km/h headwind (about 47 kW at the wheels, 48–49 kW at the motor shaft) with roughly 15% margin. It also covers 6% grade at 100 km/h (about 43 kW at the wheels).

• Engine: at 160 km/h with headwind, about 54 kW at the crankshaft, roughly 85–90% of peak torque. This is near the limit, so any speed above 160 km/h in direct mode comes from front-motor boost.

• Gear ratio: i ≈ 2.4–2.6. With i = 3, the 160 km/h point falls near 3,860 rpm and about 133 N·m, which is above this engine class’s torque capability.

• Motor speed: about 11,600 rpm at 160 km/h, so a motor rated to 16,000 rpm leaves headroom to about 220 km/h.

• Indicative electrical design: with a peak line-to-line open-circuit EMF of about 272 V at 14,000 rpm (kept under about 0.85 × the minimum DC link of about 320 V), the EMF at 160 km/h is about 225 V peak, and rated current for 55 kW is about 200 A rms. In series mode at engine speeds of 2,000–4,000 rpm, the generator runs at 5,000–10,000 rpm and the active rectifier works in boost mode, which is feasible but must be checked.

• Light-load problem: at 100–120 km/h the engine runs at only 40–55% load, which hurts efficiency. This is why load shifting (charging the battery by raising engine load) matters, as in any parallel hybrid.

10. Front axle, regeneration and battery

Regen requirement (conservative: ignores aero/rolling assistance, wheel-to-DC efficiency 0.90; front brake share 58% static plus load transfer, 62–64% at 0.2–0.3 g):

Proposed sizing (to avoid oversizing the front motor):

• Regen cap about 65 kW at the DC link. This covers 0.3 g up to about 75 km/h, 0.2 g up to about 120 km/h, and 0.15 g up to about 150 km/h. Friction brakes blend in beyond that. My estimate is that this captures roughly 80–90% of braking energy in normal driving, to be verified on real cycles.

• Front motor: about 90 kW peak, about 50 kW continuous, peak torque about 240 N·m, rated to 16,000 rpm. Motoring power is limited by the battery, not by the motor. Front-only 0–100 km/h is roughly 10–11 s.

• Front inverter: 400 V class, about 55 kW continuous and about 90–100 kW peak (motoring). That is about 280–300 A rms phase and about 270 A DC at 340 V. For regen, about 65 kW (about 160 A DC at 400 V).

• Back-EMF: the front motor’s EMF at its maximum speed (and at 160 km/h) must stay below the minimum DC-link voltage, or field weakening must be used. It is permanently coupled to the wheels.

Battery (about 11 kWh, in your 10–12 kWh range):

• The key trade-off: 65 kW into 11 kWh is about 6C. Ordinary energy-optimised LFP cells will not do that, so the regen limit depends on cell choice and temperature. If more regen acceptance is needed, raise the cell power rating rather than the front motor size.

• Cold weather: regen acceptance falls sharply (especially LFP below about 10 °C), so friction blending must always be able to cover the shortfall.

SOC strategy:

At 75% the regen headroom is about 1.65 kWh, which absorbs roughly a dozen hard stops from 100 km/h with no discharge. City driving drains it continuously, so this is adequate.

11. Braking, regeneration and long descents

• Regen is on the front axle only, the most heavily loaded and braked axle. Opening the rear contactor isolates the engine and generator so there is no uncontrolled engine braking on the rear axle.

• Engine braking is small: about 5–10 kW, around 1–2% of hard braking, so it is not relied on. At hard braking, open the contactor.

• Light and brief brake taps (a second or two at very low deceleration) can keep the link closed with fuel cut, to avoid a new synchronisation cycle. Open if braking continues.

• Long descents: if a full or cold battery limits regen, keep the link closed with fuel cut. The rear wheels then motor the engine without combustion. This resembles “B” modes in some hybrids. Friction brakes remain the backup, and the engine’s tolerance for long unfired motoring needs verification.

12. Stability and traction

• The link is stiff, and its torque is set by the engine (response in hundreds of milliseconds) and reflected inertia. ESC cannot modulate rear torque gradually in direct mode. The only control action is opening the contactor.

• Steady cruise torque is small (about 0.03 g equivalent at 120 km/h), so the real risk is a sudden disturbance, not the steady torque.

• Prevention: no linking with low grip, active ABS/ESC, tight corners, or hard acceleration requests.

• Detection: compare rear motor speed against vehicle reference speed, rate of change of speed, and phase deviation between the machines.

• Response: open the contactor (about 60–100 ms estimated, to be verified), compensate with the front motor, and inform ESC.

• Any ABS, ESC or TCS intervention, or hard braking, means open.

• The residual risk is the window between the start of a disturbance and full opening.

13. Faults and safeguards

Principles: the safe default state is “contactor open”. The link is an enhancement, not a requirement for operation. The worst assumed fault must end in a repairable state. Use ISO 26262, FMEA, FTA and fault injection. There is no absolute guarantee, because common-cause faults and systematic errors limit the achievable improvement, and probability must be measured, not assumed.

14. The contactor

• Welding is the main risk, handled by series redundancy and status sensing.

• Wear: normal operations at near-zero current cause little wear. Emergency opening under high current consumes life. The estimated count is about 30–60 thousand operations over 15 years.

• Opening policy: reduce current first (generator inverter diversion), then open.

• Type: vacuum contactor or hybrid contactor (an electronic switch in parallel for the switching instant), with documented breaking capacity for the case where the inverter fails.

• No parallel capacitor across the contacts: at these electrical frequencies it carries leakage current and can resonate. A small RC snubber only if simulation shows overvoltage.

• Monitoring: weighted event counter, contact resistance (voltage drop), coil-current signature, temperature, and an early maintenance warning.

15. Performance expectations

Estimates from engine to wheel:

• DEDEV saves about 4–5% fuel at cruise against series, and loses about 5–7% against a good parallel hybrid.

• Inverter heat falls in cruise, but cruise power is small (about 15–25 kW), so absolute losses are only about 0.3–0.6 kW per inverter. Cooling size is set by peak duty, so the benefit is mainly thermal durability on long hot trips, not smaller cooling hardware.

• DEDEV has more components and control complexity than both alternatives, and no field data.

Honest positioning: DEDEV will not beat a simple clutch in a parallel hybrid on cost or efficiency. Its niche is between range extenders and parallel hybrids, for long-distance drivers, with direct electric cruise and split drive between two axles. The benefit over a modern range extender with SiC inverters must be demonstrated by measurement.

16. Open issues and validation plan

Open: damping and pull-in torque of the twin machines, adaptive closing time, magnet tolerance to the worst-case pulse, engine tolerance to long unfired motoring, regulatory fit (ESC, braking), and the economic break-even.

Validation order:

1. Electromechanical simulation (pull-out margin, torsional dynamics).

2. Vehicle dynamics simulation with slip and ESC interaction.

3. Bench test: two machines on a common shaft with fault injection.

4. Contactor and magnet endurance tests.

5. Closed-track tests on low-grip surfaces.

6. FMEA, FTA and safety classification throughout

   ---

   
   

Gemini Layout:

# Direct Electric Drive EV system (DEDEV)

## A Unified Architecture Integrating Front-Axle Energy Recovery with Rear Direct-Electric Cruise

---

### Executive Overview

The **Decoupled Direct Electric Hybrid (DEH)** architecture combines the simplicity of direct electrical transmission with optimized thermal management and high-efficiency braking energy recovery. 

By strategically locating the internal combustion engine (ICE), generator, and main battery-electric powertrain at the front axle, while transmitting direct AC power to a matched rear motor via protected underbody cabling, the DEH system addresses key physical challenges in modern hybrid powertrains:

1. **Front-Axle Weight Transfer during Braking:** Up to 85% of kinetic energy recovery occurs at the front axle during deceleration. Placing the primary EV drive and battery interface on the front axle maximizes regenerative efficiency without compromising rear-wheel stability.

2. **Simplified Thermal Management:** Placing the ICE and generator together in the front engine bay allows direct utilization of the primary front radiator loop, eliminating long cooling lines running through the chassis.

3. **Clutchless Highway Cruising:** Power transfer during high-speed cruising occurs directly via a three-phase AC connection between the front generator and the matched rear traction motor, bypassing conventional traction inverters in the continuous propulsion path.

---

### 1. Spatial Layout & Physical Architecture

* **Front Axle & Engine Compartment:**

  * **ICE:** 1.5L naturally aspirated Atkinson-cycle engine (~74 kW peak).

  * **Generator:** Synchronous machine coupled to the ICE via a fixed step-up gear ($i \approx 2.5$).

  * **Front Traction Motor:** ~90 kW peak, ~50 kW continuous, permanently coupled to the front wheels via a fixed reducer.

  * **Battery Pack & Front Inverter:** 400V class battery (~11 kWh nominal, liquid-cooled) and a bidirectional $400\text{V}$ inverter.

  * **Cooling Loop:** Centralized front radiator cooling the ICE, generator, front inverter, and battery thermal management system.

* **Underbody Power Link:**

  * **Shielded 3-Phase HV Harness:** Heavy-duty, armoring-protected, insulated three-phase AC cabling routed through a dedicated underbody channel.

  * **Insulation Monitoring:** Continuous leakage-current detection to ensure structural and chassis safety.

* **Rear Axle Assembly:**

  * **Rear Traction Motor:** Electrically and mechanically identical "twin" to the front generator (matched pole pairs, EMF constants, and power ratings).

  * **Final Drive:** Fixed reduction ratio ($9:1$).

  * **Redundant Contactors ($K1$ & $K2$):** Series-connected electromagnetic switching assembly located adjacent to the rear motor terminals for physical link isolation.

---

### 2. Operating Modes & Dynamic Strategy

```

FRONT AXLE (Battery EV & Thermal Core)        REAR AXLE (Direct AC Shaft)

[ICE] -> [Fixed Gear] -> [Generator]          [Shielded Cable] -> [Contactors K1/K2] -> [Rear Twin Motor] -> [Final Drive]

  |                          |

[Front Inverter] <-----> [Battery Pack]

  |

[Front Motor] -> [Front Final Drive]

```

#### A. Urban / Low-Speed EV Mode ($< 70\text{ km/h}$)

* **Drive:** The vehicle operates strictly as a front-wheel-drive EV powered by the battery pack.

* **Rear State:** Contactors $K1$ and $K2$ remain **open**. The rear motor freewheels with minimal mechanicaldrag. The ICE remains off.

#### B. Direct Electric Highway Cruise ($> 70\text{ km/h}$)

* **Synchronization:** The front generator matches its frequency, phase angle, and voltage to the spinning rear motor via a brief control sequence using the armed generator-side inverter terminals.

* **Engagement:** Contactors $K1$ and $K2$ close. Direct AC current flows through the armored cabling from the front generator directly to the rear motor without passing through a continuous DC-AC inverter stage.

* **Engine-Speed Lock ($n_{\text{engine}} \propto v_{\text{vehicle}}$):**

  * $70\text{ km/h} \approx 1,750\text{ rpm}$

  * $100\text{ km/h} \approx 2,500\text{ rpm}$

  * $140\text{ km/h} \approx 3,500\text{ rpm}$

  * $160\text{ km/h} \approx 4,000\text{ rpm}$

#### C. High-Efficiency Regenerative Braking

* **Strategy:** During deceleration, contactors $K1$ and $K2$ open immediately ($< 50\text{ ms}$).

* **Front Harvesting:** The front motor operates as a high-capacity generator (capped at ~65 kW charging rate into the battery), capturing the majority of braking energy made available by forward weight transfer.

---

### 3. Functional Safety & Fault Management Framework

1. **Series-Redundant Contactors ($K1$ / $K2$):** If the primary contactor $K1$ experiences contact welding or mechanical failure, monitoring logic triggers the independent backup contactor $K2$ to isolate the high-voltage link.

2. **ESC / ABS Priority Interruption:** Active stability control triggers immediate command-opening of $K1/K2$. Torque management instantly shifts to the front axle, where high-bandwidth inverter control modulates wheel torque.

3. **Current-Diverting Disconnection:** Normal disengagement uses the generator-side active bridge to divert phase current near zero prior to opening contactor contacts, eliminating electrical arcing and extending contactor lifespan.

---

### 4. Technical Comparison

| Metric / Parameter | Conventional REEV | Parallel Hybrid (e.g., BYD DM-i) | **Decoupled DEH (Proposed)** |

| :--- | :--- | :--- | :--- |

| **Mechanical Transmission** | None | Multi-mode clutch + e-CVT gearing | **None (Fixed gear ratios only)** |

| **Cruise Inverter Path** | Continuous double-conversion loss | Direct mechanical bypass | **Direct AC electric link (No active inverter processing during cruise)** |

| **ICE & Cooling Location** | Front | Front | **Front (Optimized radiator proximity)** |

| **Braking Energy Capture** | Shared / Axle-dependent | Shared / Engine drag losses | **Front-axle focused (~80-85% kinetic recovery via front battery drive)** |

| **System Decoupling** | Integrated high-voltage DC | Mechanical-hydraulic linkage | **Fully decoupled front/rear electrical zones** |

---

### Summary Assessment

The **DEDEV Architecture** offers a balanced trade-off between mechanical simplicity and thermal practicality. By preserving the clutchless nature of electric drives, utilizing the front axle for cooling efficiency and heavy energy recovery, and using a direct AC link for highway cruising, DEH provides a distinct alternative to conventional parallel and series hybrids.