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Four stroke or two?

india Updated: Aug 27, 2009 10:17 IST
Grease Monkey

Have you ever wondered what goes on under the hood of your car? Everyone knows that fuel goes in, but how does a liquid get converted to energy that can move a car? It’s basically heat energy that makes this possible.

Your engine is a finely engineered work of art. It takes in a specific amount of air and fuel and ignites the fuel. This energy is then harnessed to make the car move. This is why automobile engines are called ‘internal combustion’ engines — ‘combustion’ is a fancy name for ‘burning’. There are other types of internal combustion engines like the jet engine, but they differ in some ways to the automobile engine. One significant difference is the fact that combustion is continuous in a jet engine, whereas in a car engine, it occurs for a fraction of the total time the engine is running.

You know that oxygen is needed for something to burn, so air is part of the mixture that goes into the engine. The option is lugging around oxygen on board like a space shuttle does, but that will be uneconomical and could be hazardous as well, so instead, we use the oxygen in the air around us. The space shuttle needs it because there’s very little oxygen in space, so all the fuel won’t do it any good without accompanying oxygen.

Men with thick glasses and white lab coats have slaved away to find the right proportion of air to fuel in the mixture that is used for combustion, and have come up with the figure ‘14.7:1’ for a mixture of air and petrol. That means for every gram of fuel that goes into the engine, there needs to be 14.7 grams of air to make the combustion perfect. This is for ideal conditions — pure fuel, pure air, etc, and this will never be found in the real world, so that figure changes ever so slightly.

However, with the help of enough sensors and computers and men in white lab coats, nothing is impossible, so today’s automobiles monitor and regulate this ratio millions of times a second, thus improving fuel economy and reducing pollution.

The most common engine type in automobiles is the ‘four-stroke’ engine. The other type is ‘two-stroke’.

Two-stroke engines have been discarded by manufacturers for the most part because their emissions are high — if you remember an old Bajaj Chetak, Yamaha RX100 or RD350, you’ll remember the blue smoke their exhaust pipes used to emit.

Users of small engines still prefer the two-stroke layout because it offers more power and less maintenance compared to a four-stroke engine. This is also why ships use two-stroke diesel engines. Two-strokers might just make a comeback, though, with advancements in technology that will allow them to meet ever-tightening emission norms.

Four-stroke engines are so named because they perform four steps: intake, compression, combustion and exhaust, over and over. The intake stroke sucks air and fuel into the cylinder, the mixture is then squeezed to a really small size and high temperature in the compression stroke, the combustion stroke burns this mixture, and what’s left over is then thrown out in the exhaust stroke.

The automobile engine is a complex piece of machinery, and the latest ones are intelligent enough to pass grade five — still, make a little effort towards understanding it; you never know when that knowledge might prove useful enough to pull you out of a sticky situation.

If you have questions or comments for Grease Monkey, email him at carsnbikes@ with Auto Tech 101 in the subject line