Between the petrol-driven vehicle and the electric automobile there is an interesting series of links provided by 'petrol-electric' systems.
At one end of the chain, electricity plays an important part in supplying power to drive the car. At the other end, electrical apparatus is introduced merely as a form of transmission gear between the petrol engine and the driving axle. The reason for attempting the petrol-electric combination will be most readily understood by considering the latter arrangement first.
The petrol engine is a high-speed engine, capable of working most satisfactorily when it runs at a uniform rate with a constant load. On the other hand, the speed of the driving axle of a car varies from a very much lower speed down to zero. It is therefore necessary, when driving a vehicle with a petrol engine, to arrange some forms of variable speed-reducing transmission gear between the engine and the driving axle. The problem is further complicated by the fact that the petrol engine is irreversible, has practically no 'starting torque,' and has a very slight overload capacity. It has to be started running 'light' and then switched on to a low gear which gives sufficient power to overcome the inertia of the car. As the speed of the car rises, there have to be successive changes of gear. These difficulties are, of course, accentuated when dealing with the heavy weight of an omnibus.
Fig. 9. Elevation and plan of a petrol-electric motor omnibus equipped by W. A. Stevens, Ltd. Directly behind the front wheels is the petrol engine, driving a dynamo through a flexible coupling. The dynamo supplies current to the motor directly behind it; and the motor drives the rear wheels through a cardan shaft. The transmission of power between the engine and the shaft is electrical at all speeds.
Practically all the troubles with petrol motor omnibuses have resided in the gear; and even the most ardent enthusiast for the all-electric faith must admit that the motor engineer has overcome these troubles (in great part if not wholly) with remarkable skill and ingenuity. But the complications of an adjustable mechanical bridge between a high-speed engine and a varying low-speed axle are so great that an electrical bridge was proposed as a substitute. By coupling the engine direct to a dynamo and by using the current so generated to drive variable-speed motors geared to the driving axle, the electrical engineer hoped to get better working results from the petrol motor than could be obtained with any mechanical transmission gear.
The most conspicuous advantage, apart from the quietness of running at all speeds, lies in the ease and smoothness with which the petrol-electric motor can start and gain speed. In this respect the combination system is practically on the same level as (or even superior to) the electric tramcar or the electric automobile. There is an entire absence of the jerks and jarring noises which usually accompany the starting of a motor omnibus. The same facility of control is of advantage in adjusting speed to suit the other traffic on the road, and also in negotiating hills.
In one class of petrol-electric vehicles the electric transmission gear is continuously used. In another, it is used at all speeds except the highest, when the engine is coupled directly (by a magnetic clutch) to a mechanical driving gear. In a third class the arrangement is more complicated, as it involves the use of storage batteries as an auxiliary to the power provided directly by the petrol engine. The Fischer type of petrol-electric vehicle uses electric transmission solely and has a fairly large battery to supplement the engine-produced current when steep hills are being negotiated. At ordinary speeds on level roads the surplus power produced by the engine goes to charge the battery.