How the engines work
The most common car power plant is the gasoline engine, also called a petrol engine or Otto engine (after the German inventor). The engine sucks in 14 to 15 parts air to one part gasoline, compresses it as the piston moves up in the cylinder, and at the top of the travel when the mixture is a tenth its original volume, the spark plug ignites the fuel-air mixture. The expanding gases force the piston down, creating power.
What was a moderately efficient process improved with fuel injection replacing carburetion, higher compression ratios (10:1), intake and exhaust valve opening adjusted to rpm and engine load (variable valve timing), and gasoline direct injection (GDI) where the fuel is squirted directly into the cylinder just before ignition, rather than injected into the intake manifolds. Turbocharging uses the force of the engine’s exhaust gases to spin an impeller which rams more air and fuel into the engine, increasing horsepower by 20%-50%. It’s like adding two more cylinders to the engine on demand, without the extra weight. The additives in premium gasoline allow for higher compression ratios and more horsepower but doesn’t directly improve fuel economy.
With a diesel engine (Rudolf Diesel, another German of the 1800s, lent his name) the air is compressed more than 20:1, raising the temperature in the cylinder to more than 1000 degrees F, then the diesel fuel is sprayed into the cylinder and is ignited by the heat. Most diesel cars and trucks have turbochargers. Diesel fuel is formulated so it ignites without the need of a spark plug. The Mazda Skyactiv-D diesel uses lower compression ratio (14:1) and other modifications to reduce emissions, lower engine cost (less reinforcement needed at 14:1), and still retain power.
A hybrid engine has both a smallish gasoline engine and an electric motor with a large battery. If the car is a strong hybrid, the electric motor can power the car by itself for a mile or two; with a mild hybrid, the electric motor only spins when the gasoline engine is running. Buyers tend to prefer strong hybrids such as the Toyota Prius over mild hybrids (GM eAssist hybrids such as the Chevrolet Malibu Eco or Hondas) because the ability to drive a mile or two on electricity is compelling and because in the past strong hybrids got better mileage.
Some hybrids use Atkinson cycle gasoline engines that make the power (downward) stroke seemingly longer than the compression (upward) stroke by not closing the cylinder’s intake valve until the compression stroke is under way. If this sounds complicated, all it means is that the engine gets to more fully burn the fuel-air mixture.
A plug-in hybrid such as the Chevrolet Volt has a much bigger storage battery and can run 30-40 miles on battery before invoking the small gasoline combustion engine. Its residual value is comparatively low, around 30% at five years, so this is not a car you buy with lowest cost in mind.
Next page: The engine break-even points…