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Anatomy Of A Four-Stroke Gas Piston Engine

  • Cutaway engine drawing, in this case a four-cylinder, 16valve, DOHC - 0
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Ever wondered how a four-stroke gas engine actually works? Wonder no longer...

The engine

The throbbing heart and soul of a classic or performance car. The basics of how your engine work are fairly simple, but both modern and classic engines have a myriad of little details that can trap the unwary, there are also some little known amazing feats that lump of metal performs which even the keenest enthusiast may find surprising.

Basic principal

Mix fuel with air and set fire to it, it goes bang and expands rather fast. Now that's fine for special effects in films but doesn't push a car along. The engine converts the explosion energy into a twisting motion that can eventually, via the gearbox, drive the wheels.

It works by keeping the explosions in cylinders, usually about the size of beer cans. It is the total capacity of all the cylinders in the engine that is referred to as the engine size or capacity. So a 2 liter four cylinder engine has four cylinders of 500cc each.

The top of the cylinder is usually sealed with the cylinder head which has valves in to let air in, and some more to let exhaust gas out. The other end of the cylinder has a piston in, a circular metal plug which is pushed down by the exploding gas.

Now if that was all there was in an engine then the explosion would just launch the piston like a cannon ball, so the piston is connected to some mechanical links, exactly like a bicycle. No, really it is. When you pedal a bike, your legs move up and down, imagine your knee is the piston and your lower leg is the connecting rod (con rod) which moves the pedals which are on a simple crank, and that's how up and down is turned in to round and round.

All the cylinders, heads and crank needs to be held in place by something pretty solid, this big lump of metal is called the engine block. Some engines separate this into two bits, the cylinders in a cylinder block and the crank in a crank case, but most have it as one unit. The bottom end with the crank in is sealed off underneath with a glorified bucket called a sump, which catches all the oil running out of all the well lubricated rotating parts, the sump also has the feed pipe for the oil pump in which sucks oil out of the sump and forces it into all the rotating parts to keep them moving freely. A dry sump has extra pumps in to force the oil into a separate tank that in turn feeds the main oil pump, this ensures there is a steady flow of oil even when cornering very hard in a racing car where the oil in the sump may slop to the side and away from a conventional pick up pipe. Oil is pumped through little tunnels or galleries in the block and head which feed the crank, con rod, cams and valve stems.

Valves

Each cylinder needs a valve to let air in from the intake system, and another one to let the burnt gas out into the exhaust system. The valves are usually like the stem of a wine glass, or a penny on a stick if you prefer. The cylinder head has holes cast through it to let gas pass into and out of the cylinder, called ports. The valves are assembled into the ports so that the valve-head blocks the port off at the cylinder end. Pushing the stick part of the valve down lifts the valve-head off the valve seat in the cylinder head so that the gas can pass. It only lifts a few mm so the shape of the valve seat makes a big difference to how much flow there is.

Some older engine designs had ports that were left with a fairly rough finish after they were cast, this roughness can reduce airflow due to turbulence so tuning shops may smooth them and possibly grind some metal away to open them up a bit, this is called porting. Oddly some engines work better with a little roughness to the port, particularly carburetted engines where fuel condenses on the port walls, a rougher surface helps break the fuel film up and mixes it with the air to make a more consistent mixture which helps make light throttle crisper and idle more stable. It's these little details that the really good tuners sort out.

The stroke

The valves have to be opened and closed at just the right point in the cycle, a four stroke engine opens the intake valve so that as the piston moves down it drags air and fuel mix into the cylinder, that's the first piston stroke. Then the intake valve is shut and the piston comes back up, this is the second stroke, squashing the mixture. If you compare the volume of the mixture at the bottom of the stroke to how small it is at the top of the stroke you usually find that its been compressed by roughly ten times, this is the compression ratio and makes a huge difference to the engine's efficiency. The compressed mixture is set fire to near the top of the stroke, which forces the piston back down on its third stroke. Then when that's done the exhaust valve opens and the piston comes back up, the fourth stroke, pushing the exhaust gas out. Then it all starts again, at full chat it might repeat 100 times a second.

In fact the speed that engines run at is worth a mention. 6000 rpm is 100 revs per second, which is 200 strokes per second. At the top of the stroke the piston stops momentarily before flying back down the cylinder, at these speeds an average car engine piston will experience a force of over one thousand G. That means a traditional car piston weighing about 500 grams actually puts a force on the con rod equivalent to half a ton, not only that but as the piston travels up and down this force goes the opposite way at the other end of the stroke, so that's half a ton of force upwards followed by half a ton downwards and repeated 200 times a second. I have been working with engines for over two decades and facts like this still impress me.

The force can be reduced by making the distance travelled by the piston shorter, and to keep the same cylinder volume the piston could be made wider, this is known as a short stroke engine and can rev higher than the same sized long stroke engine, put simply revving higher is more bangs per second and so more power. However a long stroke engine has a wider crank and just like having longer arms on a bicycle crank it is easier to pedal and produces more torque at a lower rpm.

Taken to extremes in the most powerful race engines the forces can be many tons repeated 600 times a second, in fact the old F1 turbo engines went fast enough for the piston force to exceed the force needed to buckle the con rod completely, the rods only survived because at that speed they moved out of the way fast enough not to have time to bend! In these extreme engines the rod is effectively a little bit elastic, so the compression ratio at the rev limit was higher than at idle!

I might have got slightly side tracked there, but facts like these are why I love engineering.

So there it is, the basic engine. Simple in principal but tricky in the details. Next time we look at the cam shafts, pistons and tuning.

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