by Jim Kerr
Today’s car engines put out astounding power, yet still deliver fuel economy and low emissions that were only dreams until recently. Six-cylinder engines are more powerful than V8s of a few years ago, and even four-cylinder engines are approaching the power of the last generation of eight-cylinder powerplants. How do these engines develop so much power for their size? Simple: they breathe.
The more air and fuel we put into an engine, the more power we get out.
Adding fuel is easy: just turn on the fuel injectors longer. It’s the air that’s more difficult. Sure, we can use superchargers or turbochargers to force more air in, but this adds complexity, weight and load on the engine. There are other ways to allow an engine to breathe.
Let’s start with the intake system. Air cleaner housings are now larger, and sometimes as big as the engine. This large volume helps reduce any restrictions to air flow while quieting the roar of intake air. Cooling the intake air also helps. There is typically a 1 per cent increase in engine power for every 10 degrees reduction in intake air temperature. This is almost free horsepower, but air intakes must also be designed so that they don’t let water, mud or snow into the air cleaner. The large scoops used on race cars are viable because these vehicles don’t usually run in the rain, but passenger vehicles need to be suitable for all driving conditions.
Plastic intake manifolds have allowed engineers to design long tuned runners that help pack air into the engine. Moving air has inertia, so this design creates a “ram” effect where the inertia of the moving air is used to force more air into a cylinder, even though the piston has started to move upward on its compression stroke. This principle has been used by racers for decades, but plastic now allows the manufacture of long manifold runners in a compact package that will fit under the low hood of passenger cars. Plastic has the additional benefit that it doesn’t conduct as much heat as metal does, so the intake air stays cooler.
Getting the exhaust out is equally as important. If the exhaust is restricted, fresh air can’t enter the engine. Improvements in catalytic converter construction have reduced emissions and improved the flow of exhaust. The ceramic honeycomb used to hold the catalyst material inside the converter may have as many as 900 holes per square inch. The holes are small but so is the honeycomb; there is almost no exhaust back pressure in current catalytic converters.
Exhaust pipes are carefully designed to have fewer bends, and mufflers are large to allow the exhaust gases to expand before leaving the tailpipe. This reduces noise as well as improving flow.
Engine mechanical design has improved breathing. Many engines are using three or four valves per cylinder. The valves are smaller than used on a two-valve engine, but breathing is improved because larger valves tend to block airflow where they open next to the cylinder wall. Using more valves lessens airflow restriction. Smaller valves are also lighter, so engine rpm limits can be higher before valve float occurs. Higher rpm provides more power in the same amount of time.
Almost all engines now use roller lifters or rocker arms to transfer the rotary movement of the camshaft into linear motion to open the engine’s valves. Rollers reduce friction compared to sliding components, so fuel economy improves and power increases. Rollers also allow for more radical camshaft designs, so cam lobes can now have steeper curves or ramps on the lobes to open and close valves faster and open them further. The longer an intake or exhaust valve is opened, the more air or exhaust flows through the engine and the more power is developed.
Just like an Olympic athlete, an engine needs air to perform. Athletes can learn how to breathe efficiently, and modern engines are designed to do so as well. Do it right, and both offer impressive performance. Small engines can make enough power to move large family vehicles sportily down the road, yet provide excellent fuel economy when cruising.