Getting the Most From Your Engine

by Harry Husted

Various developments have been tried to make engines run more efficiently including improving the pistons, power output, fuel consumption, reliability, and size. The basic difficulty is timing the ignition so the fuel can burn and push the piston down at the right time.

Variable Displacement Engines

Getting the fuel into the cylinder along with enough air to burn it is where the problem lies. Again, the difficulty is that the best timing for one speed is different from the best timing for any other speed. Thus valve timing has always been a compromise. If valves are timed for steady power, both top speed and output decline. Time them for racing-car performance, and the engine will barely idle.

The real difficulty lies in the size of the engine. Engines are built with a fixed displacement (the travel distance from top dead center, or when the piston is at the top of the cylinder, to bottom dead center, or when the piston is at the bottom of the cylinder) and varying the displacement has been too complex and expensive to do effectively.

Two basic ways to change engine displacement are considered: electronic and mechanical. Both work.

The idea behind the electronic approach is to electronically shut off the spark, fuel injection, and valve opening on unneeded cylinders when the car is running at light load, downhill, or cruising. The fuel savings are real in this type of environment.

Figure one below provides a visual representation of this system in action.


Figure 1 - The dual displacement engine is an important development for improved fuel economy. Only three of the engine's six cylinders operate automatically when power demands are small. All six cylinders come back into action when power demands increase.

Its electronic controls include a series of sensors: a throttle sensor, an engine speed sensor, engine phase (the order of cylinders firing) sensor, water, oil, and intake air temperature sensors. The information is fed into an 8-bit microprocessor which controls four electro-injectors that meter fuel, an electronic coil for ignition spark advance, and a throttle actuator for engine idling speed.

Varying stroke to change displacement

Mechanical means of varying engine size is another way to achieve the same results. By altering the length of the connecting rods, stroke of the engine can be lengthened or shortened, thus varying the engine's displacement. This is done by placing a control yoke with a gear connected to it between the connecting rod and the crankshaft. Figures two and three below illustrates this method.

Figure 2- Small Displacement

Figure 3- Large Displacement

The throttle control is built in such a way, to change the control yoke position by means of the control link. When the throttle control is adjusted, the control screw is turned, just like turning a screw in a nut, and the control nut moves forward. When the control nut moves forward, the control link moves with it, providing more area for the crankshaft to move, thereby, bringing the piston down farther.

Superchargers vary displacement

Fuel injection can control the volume of fuel supplied with great accuracy, but the amount of oxygen available determines how much can be burned effectively. Pumping air under pressure from either a turbocharger (an exhaust-gas driven turbine air pump) or a supercharger (a mechanically driven air pump) gets much more air into the cylinder at each stroke. This provides more fuel-burning power which pushes the piston down farther.

Car manufacturers are constantly working on providing ways to increase fuel efficiency at the same time reduce pollution. It will be interesting to see exactly how far they will go.