Engine Questions and Answers

What is the difference between a gasoline engine and a diesel engine? What is the difference between a two-stroke and a four-stroke engine? Are there any other cycles besides the Otto cycle used in car engines? How are 4-cylinder and V6 engines different? Why have eight cylinders in an engine? Why not have one big cylinder of the same displacement of the eight cylinders instead? What does 2.4 liter mean in the context of an engine? How do exhaust headers work to improve engine performance? What's the best way to increase an engine's power? •What is the difference between a gasoline engine and a diesel engine? In a diesel engine, there is no spark plug. Instead, diesel fuel is injected into the cylinder, and the heat and pressure of the compression stroke cause the fuel to ignite. Diesel fuel has a higher energy density than gasoline, so a diesel engine gets better mileage. See How Diesel Engines Work for more information. •What is the difference between a two-stroke and a four-stroke engine? Most chain saws and boat motors use two-stroke engines. A two-stroke engine has no moving valves, and the spark plug fires each time the piston hits the top of its cycle. A hole in the lower part of the cylinder wall lets in gas and air. As the piston moves up it is compressed, the spark plug ignites combustion, and exhaust exits through another hole in the cylinder. You have to mix oil into the gas in a two-stroke engine because the holes in the cylinder wall prevent the use of rings to seal the combustion chamber. Generally, a two-stroke engine produces a lot of power for its size because there are twice as many combustion cycles occurring per rotation. However, a two-stroke engine uses more gasoline and burns lots of oil, so it is far more polluting. See How Two-stroke Engines Work for more information. •Are there any other cycles besides the Otto cycle used in car engines? The two-stroke engine cycle is different, as is the diesel cycle described above. The engine in the Mazda Millenia uses a modification of the Otto cycle called the Miller cycle. Gas turbine engines use the Brayton cycle. Wankel rotary engines use the Otto cycle, but they do it in a very different way than four-stroke piston engines. How are 4-cylinder and V6 engines different? The number of cylinders that an engine contains is an important factor in the overall performance of the engine. Each cylinder contains a piston that pumps inside of it and those pistons connect to and turn the crankshaft. The more pistons there are pumping, the more combustive events are taking place during any given moment. That means that more power can be generated in less time. 4-Cylinder engines commonly come in “straight” or “inline” configurations while 6-cylinder engines are usually configured in the more compact “V” shape, and thus are referred to as V6 engines. V6 engines have been the engine of choice for American automakers because they’re powerful and quiet but still light and compact enough to fit into most car designs. •Why have eight cylinders in an engine? Why not have one big cylinder of the same displacement of the eight cylinders instead? There are a couple of reasons why a big 4.0-liter engine has eight half-liter cylinders rather than one big 4-liter cylinder. The main reason is smoothness. A V-8 engine is much smoother because it has eight evenly spaced explosions instead of one big explosion. Another reason is starting torque. When you start a V-8 engine, you are only driving two cylinders (1 liter) through their compression strokes, but with one big cylinder you would have to compress 4 liters instead. What does 2.4 liter mean in the context of an engine? When the piston moves from top to bottom, it sucks in a certain amount of air. How much air it can suck in depends on how big around the piston is, and how far it moves when it goes from top to bottom. Let's say that the piston in your car is 4 inches (10.16 centimeters) in diameter (also known as the bore), and it moves 4 inches from top to bottom (also known as the stroke). That means that one piston in your engine can suck in: 5.08 cm (bore/2) ^ 2 * 3.14 * 10.16 cm (stroke) = 823.3 cubic centimeters If your car has 4 cylinders, then it has a has a total displacement of: A car manufacturer would round that up and say that your car has a 3.3-liter engine. This means that the displacement of this particular engine is 3.3 liters. If you were to turn the crankshaft of this engine through two complete revolutions, the four pistons would inhale a total of 3.3 liters of air. So why should you care, and why is there a sticker on the back of many cars telling you the engine's displacement? The displacement of the engine gives you some estimate of the maximum power that the engine can produce. When you mix gasoline with air and burn it in an engine, you can only mix in so much gasoline. The amount of gasoline is limited by the amount of oxygen -- if you mixed in more gasoline, it would not matter because there would be no oxygen in the cylinder to burn it. The ratio is about 15 to 1 -- that's 15 parts air to one part gasoline, by weight. The displacement tells you the maximum amount of gasoline the engine can burn, and this controls the maximum power the engine can produce. Of course, it is possible to create a 10-liter engine that has really bad performance. And it is also possible to create a 1-liter engine that is highly tweaked to give absolutely the best performance possible from the gasoline it receives. And these two engines could have the same horsepower rating even though one has 10 times more displacement than the other. As a general rule, though, a 10-liter engine should generate 10 times more power than a 1-liter engine if everything else is equal. How do exhaust headers work to improve engine performance? Headers are one of the easiest bolt-on accessories you can use to improve an engine's performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders. The gasoline in the cylinder burns and expands during this stroke, generating power. The other three strokes are necessary evils required to make the power stroke possible. If these three strokes consume power, they are a drain on the engine. During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger. In a normal engine, once the exhaust gases exit the cylinder they end up in the exhaust manifold. In a four-cylinder or eight-cylinder engine, there are four cylinders using the same manifold. From the manifold, the exhaust gases flow into one pipe toward the catalytic converter and the ­muffler. It turns out that the manifold can be an important source of back pressure because exhaust gases from one cylinder build up pressure in the manifold that affects the next cylinder that uses the manifold. The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector. What's the best way to increase horsepower? There are many different ways to create more horsepower from a stock engine. Here are several examples (generally in order from least to most difficult/expensive): ­Change your computer chip. Sometime­s, but certainly not always, you can change a car's performance by changing the ROM chip in the engine control unit (ECU). Let air come in more easily. As a piston moves down in the intake stroke, air resistance can rob power from the engine. Some newer cars are using polished intake manifolds to eliminate air resistance there. Bigger air filters and reduced intake piping can also improve air flow. ­Let exhaust exit more easily. If air resistance or back-pressure makes it hard for exhaust to exit a cylinder, it robs the engine of power. If the exhaust pipe is too small or the muffler has a lot of air resistance then this can cause back-pressure. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system. Change the heads and cams. Many stock engines have one intake valve and one exhaust valve. Buying a new head that has four valves per cylinder will dramatically improve airflow in and out of the engine and this can improve power. Using performance cams can also make a big difference.