WO2007135428A1

WO2007135428A1 - A powertrain layout for a hybrid road vehicle

Info

Publication number: WO2007135428A1
Application number: PCT/GB2007/001905
Authority: WO
Inventors: Chistopher William Henderson Ellis
Original assignee: Hykinesys Inc.
Priority date: 2006-05-23
Filing date: 2007-05-22
Publication date: 2007-11-29
Also published as: GB0610177D0

Abstract

The invention relates to a novel power train layout (10) for a hybrid road vehicle, in which a rear mounted internal combustion engine (12) and a front mounted surge power unit (52, 60,62,63) are arranged to drive the front wheels (14,16) only of the vehicle. The novel layout allows for a reduction in height of the bonnet and roof lines of the vehicle which accordingly reduces aerodynamic drag of the vehicle, and improves fuel consumption. Further aerodynamic and fuel consumption gains are made by fitting the vehicle with a smooth under tray, which is made possible by the absence of an exhaust system spanning the underside of the vehicle.

Description

A POWERTRAIN LAYOUT FOR A HYBIOD ROAD VEHICLE

This invention relates to a power train layout for a hybrid road vehicle, and a hybrid road vehicle incorporating such a power train layout.

Over 70% of the world's current production of cars have front engines which drive the front wheels, and which are consequently described as front-wheel-drive (FWD) vehicles. Most other road vehicles have front engines driving the rear wheels (rear- wheel-drive or RWD), although there are an increasing number of cars and light trucks which drive through all four wheels, some or all of the time. Finally, there are relatively small numbers of sports cars with rear- or four-wheel-drive with engines just in front of the rear axle (so- called mid-rear engine) or behind it (rear-engine).

A hybrid vehicle typically combines powerful regenerative braking and an internal combustion engine (ICE), and is designed to reduce energy consumption at both low and high speeds. Almost every conceivable combination of engine location and driven axles is being used, except the combination of a rear- or mid-rear engine driving only the front wheels. Such a configuration made little sense when engines were a major proportion of total vehicle weight because the static weight distribution might be 40:60 front-rear which would get even worse (due to weight transfer to the rear) on acceleration, resulting in a loss of traction and steering control. Consequently, all current rear and mid-engined cars drive either the rear wheels only or all four wheels. However, the 'missing layout¹ is ideal for mass market hybrid passenger cars, vans and SUVs (Sport Utility Vehicles), as will be shown.

Relative to conventionally powered vehicles, the novel rear-engine front-wheel-drive layout for a hybrid results in reductions in fuel consumption which are beyond the reach of any other hybrid layout. The two main reasons are less aerodynamic drag and a smaller, lighter and consequently more economical engine.

One widely used legal definition of 'hybrid vehicle' is:- "A motor vehicle that draws propulsion energy from on board sources of stored energy that are both an internal combustion or heat engine using combustible fuel and a rechargeable energy storage system."

The first mass-production hybrids were FWD (front-engine, front-wheel-drive). Their four cylinder engines were mounted transversely, attached to novel transmissions with integrated electric motor/generators, fed by battery packs fitted behind the seats. The substantial extra weight of the battery packs was carried mainly by the rear wheels, which had the beneficial effect of moving the weight distribution towards the handling ideal of

50:50 and away from the typical 60:40 of most conventional FWD cars. Then came the first four-wheel-drive hybrids, but these models were also available in FWD form, at a lower price. The first production RWD hybrids became available soon after.

To achieve excellent fuel consumption at freeway/motorway speeds it is essential to minimize aerodynamic drag. Even at a relatively conservative speed such as 70 mph, aerodynamic drag can be absorbing more than two thirds of the energy supplied by the engine which reaches the driving wheels. Because aerodynamic drag rises with the square of the speed and the other major losses tend to rise linearly, the proportion of total energy lost to aerodynamic drag rises at higher speeds, to become the dominant factor at motorway speeds.

Aerodynamic drag is essentially determined by two main factors, the frontal area of the vehicle (the maximum cross-sectional area) and the coefficient of drag, or Cd. The Cd is a measure of the sleekness of the vehicle, mainly a function of the shape. Typically a large SUV will consume much more fuel on the motorway as a saloon car of the same weight travelling at the same 70 mph. The frontal area of the SUV might be 50% greater, and its Cd might be 0.40 versus 0.26 for the car. Multiply the two factors together and the result is a doubling, at least, of aerodynamic drag, resulting in a significant increase in fuel consumption.

During most of the history of the motorcar, the overall height of the vehicle fell gradually, as model succeeded model. Recently, however, there has been a tendency for heights to rise again. One reason is a new European regulation designed to give pedestrians greater protection in an accident. It requires that there must be adequate space between the underside of the bonnet and the top of the engine, which is having the effect of raising the bonnet line of most new designs intended for the European market. Unless engine heights can be lowered or engines moved to the rear, increased protection for pedestrians will indirectly cause fuel consumption to be higher than it otherwise need be. Removing the engine from the front of the vehicle would allow the bonnet line, and hence the height of the roof, to be lower relative to the current norm.

A further source of aerodynamic drag is from the rough underfloor surfaces of most cars, SUVs and vans. Much of this is caused by the need to cool the exhaust system extending the length of the vehicle, and to cool the vehicle structure around the exhaust system. Moving the engine to the rear allows a smooth under tray to be fitted, reducing the Cd substantially. Consequently, there is a strong case for rear-engined passenger vehicles, if a solution to the problem of weight distribution can be found.

Regenerative braking, in which an electrical generator converts the kinetic energy from the braking force into electrical energy for later use, operates most effectively if applied to all four wheels, as is the case with conventional braking, and for the same reasons. On most vehicles, the front brakes are larger and more powerful than those at the rear, because weight transfer forward during braking means that more of the vehicle's kinetic energy can be extracted and transferred to air by the front brakes than the rear brakes, provided the front brakes are sufficiently powerful. Consequently, if cost considerations force regenerative braking to be available only on the wheels of one axle, then it should be on the front wheels.

hi summary, if cost is not the key consideration, then a hybrid should have both braking and accelerating power available at all four wheels. However, if only one drive axle can be afforded then it should be the front. So the market for hybrids will divide very much along conventional lines, except that RWD only hybrids will be relatively rare. The mass market, say 80% of all new hybrid cars, will be FWD, as is the case already. The luxury, sporting and heavy duty segments will be largely four-wheel drive.

Conventionally, if a vehicle is front-wheel drive, then the engine's weight should be over the driving wheels to ensure good traction and minimize undesirable traits like torque steer. However, the essential characteristic of a hybrid is that the two key functions of a conventional engine, the efficient supply of sufficient power for continuous cruising and extra power for intermittent bursts of acceleration, are separated out into two units which are typically termed the Surge Power Unit (SPU) and the Energy Supply Unit (ESU). The Surge Power Unit (SPU) or energy storage and drive arrangement provides powerful acceleration when intermittently required, and also hosts the new complementary function of regenerative braking and the temporary energy store needed to hold the saved energy between braking and accelerating. The Surge Power Unit may be an electric motor/generator connected to a suitable rechargeable storage device for electric energy, such as one or more battery or capacitor packs, or a combination of these. Alternatively the SPU may be a kinetic energy storage and drive assembly. The Energy Supply Unit (ESU) can, in principle, be a conventional combustion engine, fuel cell stack, a gas turbine or a Stirling engine. It is likely that for the immediate future the ESU will be a conventional combustion engine. In this specification the term "power unit" is intended to generically encompass these and other ESU options. The ESU's role is to convert fossil or biofuel as efficiently as possible at the 'running average rate¹ required by the vehicle, to keep the SPU well stocked with energy, replacing the various losses in the system.

An obvious first reaction, typified by the first generation of production electric hybrids, is to leave the engine and fuel tank of the ESU in their conventional positions, integrate the motor/generator(s) with the differential, etc, at the front of the vehicle, and balance the weight distribution by putting most of the rest of the SPU (i.e. the battery) behind the rear seats, directly over the rear wheels.

However, this misses a major opportunity to radically reduce fuel consumption at motorway and dual carriageway speeds. Because the engine is at the front, the hot exhaust pipe and associated catalytic converters and silencers have to be accommodated down the centreline of the vehicle. A really low Cd is inhibited by all the 'hot plumbing' that must be left exposed to air, to cool the exhaust itself and to prevent heat from being conducted into the cabin and increasing the load on the air conditioning system.

Major reductions in aerodynamic drag can be achieved by either doing without an engine, as in the GM EVl (Cd 0.19), or mounting it in the rear, as in the GM Precept (Cd 0.163). The fuel consumption of the global car fleet could be radically reduced if the average Cd could be reduced from its current value of over 0.30 to, say, 0.20, and the average frontal area reduced by 15%. The features of this innovation make this possible in low-cost hybrid cars and light vans. The optimum configuration for a front-wheel-drive hybrid vehicle is to put the engine behind or just in front of the rear axle, and place the energy store and hybrid drive up front, their weight enhancing the traction of the front wheels.

An object of the present invention is to provide an improved hybrid drive train for the majority of road vehicles which will help reduce fuel consumption at all speeds.

According to the invention there is provided a hybrid road vehicle having: a body having a front and a rear; a front wheel assembly having opposing driven front wheels; a rear wheel assembly having at least one rear wheel; a rechargeable energy storage and drive arrangement positioned proximate to said front wheel assembly to the front of the vehicle body; a power unit positioned proximate to said rear wheel assembly to the rear of the vehicle body; and a transmission system including a forwardly extending propeller shaft connecting said power unit to said front wheel assembly, said transmission system arranged to transmit power from either or both of said rechargeable energy storage and drive arrangement and said power unit to drive said front wheels of the vehicle.

The invention further provides a drive system for a hybrid road vehicle comprising: a front drive assembly having drive shafts for connection to opposed front wheels; a forwardly mounted energy storage and drive arrangement positioned proximate to said front drive assembly; a power unit positioned reawardly of said front drive assembly; and a transmission system including a forwardly extending propeller shaft connecting said power unit to said front drive assembly, said transmission system arranged to transmit power from either or both of said rechargeable energy storage and drive arrangement and said power unit to drive said front drive assembly. The power unit may comprise an internal combustion engine, a fuel cell stack, a gas turbine, Stirling engine, or any other such device configured to convert energy in the form fossil or biofuel into a form suitable to drive the vehicle. In a first embodiment of the proposed solution a rear mounted internal combustion engine is connected to drive the front wheels through a forwardly extending propeller shaft, a differential, and a driveshaft connected to each wheel. The internal combustion may be positioned so that it sits behind the rear axle of the associated vehicle (rear engined), or just in front of the rear axle (mid- rear engined). The weight of the internal combustion engine in either case is centred at the rear of the drive train and its associated vehicle. The internal combustion engine may be longitudinally or transversely mounted and inclined to the vertical to minimise intrusion into the vehicle body. The internal combustion engine may drive the front wheels via any suitable transmission; possible transmission options include manual, fully automatic, automated manual or continuously variable.

At the front of the drive train, positioned forward of the driveshafts, is the energy storage and drive arrangement. In a first embodiment, this may comprise a kinetic energy storage and drive assembly. Preferably, the kinetic energy storage and drive assembly is in the form of twin contra-rotating rotors, connected via a transfer box which ensures that the rotors always contra-rotate at the same speed. The kinetic energy storage and drive assembly is able to drive the front wheels via the differential and driveshafts, but preferably may also recover and store energy by regenerative braking of the front wheels. Preferably, the kinetic energy storage and drive assembly drives the front wheels via a continuously variable transmission. Instead of kinetic energy storage, a suitable storage device for electrical energy may be employed.

In another embodiment, an internal combustion engine is rear mounted and drives the front wheels in the same manner as the first embodiment. In this embodiment the energy storage and drive arrangement, which again drives the front wheels, is in the form of an electric motor which is fed with energy from a storage device comprising one or more battery or capacitor packs. The storage device is mounted at the front end of the vehicle to counterbalance the weight of the rear mounted internal combustion engine and provide as close to 50:50 weight distribution as possible. Preferably, the electric motor also acts as a generator and feeds energy to the storage device by regenerative braking. Preferably, the storage device is placed forwardly of the driveshafts to improve weight distribution at the front end. The storage device could of course comprise several individual units arranged around the vehicle for packaging purposes.

The effect of mounting the internal combustion engine at the rear of the vehicle, as in both preferred embodiments, is that the overall height of the vehicle, for reasons explained above, is reduced thereby improving the aerodynamics and reducing the fuel consumption of the vehicle. Furthermore, by driving the front wheels only, a cost benefit is gained by avoiding the complexity and expense of four wheel drive, and at the same time provides consumers with the familiar handling characteristics of a front wheel drive vehicle.

Further features of the invention will appear from the following description of embodiments of the invention and will now be more particularly described by way of example with reference to the accompanying drawings, in which: Figure 1 shows, in perspective, a typical example of the power train in a hybrid family car;

Figure 2 is a perspective view of a four-wheel-drive vehicle equipped with a hybrid power train comprising a kinetic energy storage and drive assembly and a conventional reciprocating engine and its gearbox; Figure 3 is a perspective view of the power train of a front-wheel-drive family car with a rear-mounted reciprocating engine and a front-mounted kinetic energy storage and drive assembly and kinetic hybrid drive;

Figure 4 is a perspective view of the power train of a front-wheel-drive family car with a rear-mounted reciprocating engine and front-mounted electric motor/generator and electric battery based hybrid drive;

Figure 5 is a side view comparing the body profile of a conventional hybrid vehicle to that of a hybrid vehicle incorporating the power train layout of the present invention.

A typical hybrid power train layout 10 is shown in figure 1. The power train 10 consists of an internal combustion engine 12 transversely mounted at a front end 6 of the power train, a gearbox 13 connected to the internal combustion engine 12 and a differential 18 which provides drive to a first driveshaft 20 and a second driveshaft (not shown) for driving front wheels 14 and 16 respectively. The power train also includes one or two electric motors/generators (not shown) also mounted at the front end 6 of the power train, and which also drive the front wheels 14 and 16. The electric motor/generator is connected to electric battery pack 22 which is situated at a rear end 8 of the power train 10, between rear wheels 24 and 26. It can be appreciated from figure 1 that the majority of the power train components are positioned at the front end 6 of the vehicle, and that the majority of the weight of the power train therefore acts on the front wheels 14 and 16 which may result in poor ride and handling characteristics.

Figure 2, in which components common to figure 1 are identified with like reference numerals, shows a comprehensive but expensive hybrid power train layout in which all four wheels 16, 24 (opposing wheels omitted for clarity) are driven by a kinetic energy storage and drive assembly 28 and a rear-mounted internal combustion engine 12. This layout improves weight distribution between the front end 6 and rear end 8 compared to the layout shown in figure 1, as well as the additional benefits of four wheel drive being available for both accelerating and regenerative braking. In this case the kinetic energy storage and drive assembly 28 is in the form of twin contra-rotating rotors, comprising front rotor 30 (shown without casing), and rear rotor (not shown) contained within casing 32. The two rotors are offset to allow for a gear connection within transfer box 34 to ensure that the two rotors always contra-rotate at the same rotational speed. The front rotor 30 is linked to and drives the front wheels via planetary gearbox 36, continuously variable transmission 38, differential 40 and driveshafts 20 and 21. Energy is carried to and from the front rotor along this path. Likewise, the rear rotor is linked to the rear wheels by continuously variable transmission 42, transaxle 44 and rear driveshafts 46 and 47, and energy is carried to and from the rear rotor along this path.

The internal combustion engine 12 is longitudinally mounted at the rear of the vehicle. Drive from the internal combustion engine is supplied to the rear wheels via transaxle 44 and driveshafts 46 and 47, and to the front wheels through the rotor assembly of the kinetic energy storage and drive assembly This set-up, whilst offering the aerodynamic advantages of having a rear mounted engine, is complicated and potentially prohibitively expensive for the mass market. A first embodiment of a preferred and innovative solution is shown in figure 3, in which components common to the earlier figures are given like reference numerals. A rear mounted internal combustion engine 12 drives the front wheels via a forwardly extending propeller shaft 50. The energy storage and drive assembly comprises a kinetic energy storage and drive assembly 52 which is mounted at the front end 6 between the front wheels, to provide a weight distribution as close to 50:50 as possible. The internal combustion engine 12 is inclined to minimise intrusion into the boot area of the vehicle. Drive from the internal combustion engine is fed to the front wheels only, through gearbox 54, propeller shaft 50, a differential (not shown), and to the front wheels through driveshafts 20 and 21.

The kinetic energy storage and drive assembly 52 consists of twin contra-rotating rotors contained within casings 56 and 57 respectively, and mounted transversely relative to the propeller shaft 50. The rotors are offset and operate in the same manner as those described in figure 2. Drive from the kinetic energy drive and storage assembly is fed to the front wheels via continuously variable transmission 58, differential (not shown) and driveshafts 20 and 21. Energy is transferred to the storage device along this same path by means of regenerative braking.

A second embodiment of the preferred solution is shown in figure 4, in which components common to the earlier figures are given like reference numerals. The kinetic energy drive and storage assembly 52 has been replaced by an electric motor/generator 60 fed by battery packs 62 and 63. Drive from the internal combustion engine is fed to the front wheels by forwardly extending propeller shaft 50 and the front driveshafts as explained above. The electric motor/generator 60 can draw energy from the battery packs 62 and 63 to drive the front wheels, or when in its energy generating mode can supply energy back to the battery packs 62 and 63 by means of regenerative braking.

It will be appreciated from figures 3 and 4 that since the internal combustion engine is at the rear end 8 of the power train, the exhaust pipe components (not shown) will not span the entire length of the underside of the associated vehicle therefore allowing a smooth under tray to be fitted, resulting in an improved drag coefficient. Whilst the propeller shaft 50 does span this length it requires less cooling and insulation clearance. The aerodynamic advantages of the arrangement shown in figures 3 and 4 i.e. a rear mounted engine driving the front wheels, is best seen in figure 5 in which components common to the earlier figures are given like reference numerals.. This figure shows a first hybrid vehicle 70 having a front mounted internal combustion engine 12. Whether the secondary power source is front or rear mounted is not critical since it is the internal combustion engine 12 which is the taller of the two, and therefore the limiting factor in trying to minimise vehicle frontal area. To meet pedestrian safety guidelines there is necessarily a clearance C between the underside of the bonnet 72 and the top of the engine 12. The bonnet line therefore extends from a lower height Bl above the ground 78 to an upper height B2 where it meets the vehicle windscreen 74. The roof 76 is at a height Rl above the ground 78. The minimum height of the roof R2 is limited by the upper height of the bonnet B2, to provide for driver and passenger visibility and headroom.

A second vehicle 80 is shown on the same ground level 78 as the first vehicle 70. The second vehicle 80 has the same hybrid power train layout as that of the preferred embodiments shown in figures 3 and 4 i.e. with an internal combustion engine 12 at the rear of the vehicle and a secondary power source 82 (be it a kinetic energy storage and drive assembly or an electric motor/generator) at the front of the vehicle. Because of the reduced dimensions of the secondary power source 82 in relation to the internal combustion engine 12, the lower and upper bonnet levels B3 and B4 are lower than Bl and B2 whilst still retaining the necessary clearance C beneath the bonnet 84. Because of the lower height B4 of the bonnet 84 of vehicle 80 compared to the bonnet height B2 of vehicle 70, the roof 88 of vehicle 80 has a height R2 above the ground which is accordingly a height H lower than roof line Rl, whilst the dimensions of the windscreen need not be reduced.. Headroom levels are also kept constant, as the seats may be mounted lower down in the vehicle as a consequence of the lower height B4. The reduction in roof height leads to a lower frontal area for vehicle 80, which results in a reduction in aerodynamic drag and improved fuel consumption.

Claims

1. A hybrid road vehicle having: a body having a front and a rear; a front wheel assembly having opposing driven front wheels; a rear wheel assembly comprising at least one rear wheel; a rechargeable energy storage and drive arrangement positioned proximate to said front wheel assembly and to the front of the vehicle body; a power unit positioned proximate to said rear wheel assembly and to the rear of the vehicle body; a transmission system comprising a forwardly extending propeller shaft connecting said power unit to said front wheel assembly, said transmission system arranged to transmit power from either or both of said rechargeable energy storage and drive arrangement and said power unit to drive said front wheels of the vehicle.

2. A vehicle as set forth in claim 1 wherein the rechargeable energy storage and drive arrangement comprises a kinetic energy storage and drive arrangement.

3. A vehicle as set forth in claim 1 wherein the rechargeable energy storage and drive arrangement comprises a storage device for electrical energy and an electric motor.

4. A vehicle as set forth in any preceding claim wherein the rechargeable energy storage and drive arrangement has a regenerative braking function.

5. A vehicle as set forth in any preceding claim wherein the transmission system includes a continuously variable transmission arrangement.

6. A vehicle as set forth in any preceding claim wherein said body includes bonnet having a bonnet line and a roof having a roof line, said bonnet line positioned a first height above a ground level and said roof line positioned a second height above the ground level, the positioning of said internal combustion engine at the rear of the vehicle enabling said first and second heights to be minimised, so as to reduce the frontal area and aerodynamic drag of the vehicle.

7. A vehicle as set forth in any preceding claim having a smooth under tray to reduce the aerodynamic drag of the vehicle.

8. A drive system for a hybrid road vehicle comprising: a front drive assembly having drive shafts for connection to opposed front wheels; a forwardly mounted energy storage and drive arrangement positioned proximate to said front drive assembly; a power unit positioned rearwardly of said front drive assembly; and a transmission system including a forwardly extending propeller shaft connecting said power unit to said front drive assembly, said transmission system arranged to transmit power from either or both of said rechargeable energy storage and drive arrangement and said power unit to drive said front drive assembly.

9. A drive system as set forth in claim 8 wherein the rechargeable energy storage and drive arrangement comprises a kinetic energy storage and drive arrangement.

10. A drive system as set forth in claim 8 wherein the rechargeable energy storage and drive arrangement comprises a storage device for electrical energy and an electric motor.

11. A drive system as set forth in any of claims 8 to 10 wherein the rechargeable energy storage and drive arrangement has a regenerative braking function.

12. A drive system as set forth in any of claims 8 to 11 wherein the transmission system includes a continuously variable transmission arrangement.