How Your Engine Works

Your engine is a complex piece of equipment that most motorists take for granted and tend to ignore until it goes wrong. Then the motorist cusses and complains about how much time and/or money it has to be invested to get the engine repaired. If you know a little bit more about what is happening inside the engine and how it creates the power that you ask of it by pressing the accelerator then you may better appreciate why it is important to observe a maintenance schedule to look after the engine.

Basic Identification

Identifying various components around the engine will enable you to communicate with other people which area of the engine you are talking about and you will also understand them when they are talking to you. Below is a simplified version of the engine that is in most cars. Whilst engine design may change with regards to the number of cylinders or the configuration of those cylinders (how they sit within the engine bay of the car) such as a V8 or a twin opposed, the basics of what you see inside and how they function are pretty much the same. For the technical amongst you the engine below is a straight four, four stroke, single overhead cam (SOHC). There are many variants and you can find out more here.

Camshaft

Timing Belt

Rocker Arm

Valve

Valve Spring

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Cylinder head

Head Gasket

Cylinder Block

Base Gasket

Piston

Crankcase - The crankcase serves several functions. It provides a housing for the crankshaft to rotate in whilst also serving as the base for the rest of the engine components. It also provides a splash pool of oil, some of which is pumped round the engine, the rest literally splashes over internal engine components. The crankcase is often call the sump.

Crankshaft - The crankshaft takes the linear motion of the pistons pushing down on it and changes it to rotary motion which, after passing through other devices on the car like the clutch and the gearbox, can be fed to the wheels to make them go round.

Base Gasket - The base gasket forms a liquid tight seal between the cylinder block and the crankcase, keeping fluids such the the oil and the coolant in their respective locations. Unlike the head gasket it does not have to operate at such high pressures since there is no compression or combustion taking place around it.

Connecting Rod - The connecting rod provides a flexible link between the piston and the crankshaft and enables the power generated from combistion pushing on the piston to be transmitted to the crankshaft.

Cylinder Block - The cylinder block contains the cylinders, inside which each piston slides. The top part of each cylinder, along with part of the cylinder head, has a sealed space where the power is generated and this is called the compression chamber. There is also space for timing gear linking the crankshaft to the camshaft and oilways and coolantways to help lubricate and cool the engine.

Piston - The piston pushes down on the connecting rod when the fuel explodes forcing the crankshaft to turn.

Piston Rings - Piston rings form an almost airtight seal between the moving component of the piston and the cylinder block. Without the piston rings no compression would be made, no rapid combustion would occur and most of the power generated from whatever combustion did occur would slide past the piston generating no movement in the crankshaft.

Compression Chamber - The compression chamber is the almost airtight volume of space between the cylinder head and the piston where fuel is brought into the engine from the inlet valve, squeezed into a very small space increasing it's temperature and explosive capability, ignited and then the exhaust gases are removed from the chamber.

Head Gasket - The head gasket provides an airtight seal between the cylinder head and the cylinder block. Without it the high pressures of compressed fuel in the compression chamber and the lower pressures of the lubricating oil and coolant would leak and either cross-contaminate each other or vent into the outside world, causing additional pollution.

Cylinder Head - The cylinder head, as well as providing a top cap for each of the cylinders enabling the compression of fuel prior to combustion and the subsequent power derivation from the combustion to push on the pistons, also provides a mounting place for all the valve and rocker gear.

Camshaft - The camshaft controls when fuel is let into each cylinder and when the exhaust gasses are let out by pushing on the rocker arms.

Rocker Arms - The rocker arms are pushed by the camshaft and, in turn, push on the valve. There is, generally, one rocker arm per valve.

Valve - The valve, when pushed by the rocker arm, opens and either lets fuel into the cylinder or exhaust gasses out. There are a minimum of two valves per cylinder, one for fuel feeding in to the cylinder and one for letting the exhaust gasses out.

Valve Springs - The valve spring closes the valve, preventing the movement of gasses, when the pressure is removed from the rocker arms.

Timing Belt/Chain - The timing belt or chain co-ordinates the rotation of the camshaft with the rotation of the crankshaft. Without the two rotating in sync gasses would not get in or out of the engine at the right times.

Crankcase

Crankshaft

Piston Rings

Connecting Rod

Simple Operation

Using Video 1 below to show the explanation, the engine is initally turned over by a starter motor (not shown) that starts to rotate the crankshaft. At this time two events now take place at the same time.

The piston is moved up and down by pushing and pulling on the connecting rod.
The timing belt rotates the camshaft at a different speed to the crankshaft (usually half the speed in a SOHC engine).

The camshaft then pushes down with a cam on the inlet valve, just as the piston starts to drop from it's maximum height (known as Top Dead Centre - TDC). Because the compression chamber is almost airtight and the inlet valve is the only component of the engine that enables air to get into the chamber because it is open a mixture of fuel and air is either drawn from the carburettor, as with the engine below, or squirted in by a fuel injector into the compression chamber as the piston moves down the cylinder trying to create a vacuum.

The crankshaft and camshaft continue to rotate in the same direction and the inlet valve closes as the cam stops exerting pressure on the valve and the valve spring pushes the valve back into its seat. This occurs just as the piston reaches it's minimum height (known as Bottom Dead Centre - BDC) and the motion from TDC to BDC is known as the downstroke.

As the piston starts travelling back up the cyclinder (on the upstroke), because the compression chamber is practically airtight now with the valves closed, the fuel/air mixture is compressed into a very small space at the top of the cylinder which both heats it up and also makes it highly volatile.

As the piston reaches TDC again the ignition of the fuel/air mixture takes place, either by a spark plug in a petrol car or self-combusting in a diesel, causing a small explosion in the compression chamber. This explosion creates a greater volume of exhaust fumes which then pushes the piston on its downstroke back to BDC.

At this point the starter motor usually disengages and the momentum of the crankshaft keeps the engine in motion.

As the piston reaches BDC the camshaft rotates a cam to push on the exhaust valve, opening it. As the piston travels on it's upstroke to TDC it pushes the exhaust fumes out and, as it reaches TDC, the exhaust valve closes using the valve spring to seat it. The crankshaft and camshaft continue to rotate and the inlet valve is opened again and the process repeated.

By having the above events occur at staggered, but even, intervals on all four cylinders (see Video 2) in the engine enables a smooth output of rotary power to be developed which, when outputted from the crankshaft to the clutch and gearbox, eventually ends up rotating the wheels of the car, pushing the car forward or backward. Explanations of the clutch and gearbox function will be forthcoming.

When you press down on the accelerator more of the fuel/air mix is allowed into the combustion chamber which creates a big explosion. In turn this moves the piston a little faster which pushes the crankshaft faster. When you lift your foot off the accelerator this restricts the fuel/air mix which, in turn, slows the engine.

With all these pieces of metal moving over or through each other, just like when you rub your hands together, a lot of heat is generated. When anything heats up it swells which, in an engine, would be catastrophic as the engine would seize and stop moving. So it is important that oil is kept flowing around the engine to both reduce the friction between two sliding surfaces, like margarine in your fingers, and also to convey the heat build up away from the engine to reduce how much all the engine components swell. If you have too little oil in your engine then the components cannot be effectively lubricated. Too much oil in the engine will mean that you will over pressurise the crankcase and blow a gasket. The only way for a gasket to be repaired is for the engine to be stripped apart, the old gasket removed, and a new gasket put in place. So, when you check your oil level with the dipstick it is very important that the oil level be between the minimum and maximum marks.

Compression Chamber

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