Tech Talk
Four-Stroke Engine Theory

The Four-Stroke Engine

by Brian Pollard

The four stroke engine has been around for a very long time and is well understood. It produces lots of torque and low down power with the minimum of fuss. It doesn’t like to rev very high, due to the valves mechanically opening and more often than not shutting with spring devices, but all in all is an easy engine to work on and improve.

The four strokes are... The whole cycle now repeats, sometimes at around 6-7000 revolutions per minute.

This is an oversimplification of each stroke as there are other things happening as the piston moves up and down during its cycle.

For instance, as the exhaust stroke is near completion the inlet valve starts to open to take full advantage of the next downward stroke of the piston. Any overlap of the valves opening or closing during an opposite cycle is called valve overlap. Think of overlap helping the parts to be ready for action when called upon. All mechanical movement takes time and this overlap helps to reduce waiting time when demanding action of valves, springs, rockers, pushrods, cams etc etc. Another advantage of valve overlap is the power advantage gained by opening the inlet valve early and keeping it open as long as possible before compression pushes any fuel/air back out of the port. Similarly if the exhaust valve is opened quickly and long enough then all waste gases will be drawn into the exhaust before the inlet valve opens and meets this gaseous blockage.

Manufacturers of cams know the effect of producing cams with sufficient overlap to work efficiently but they choose to make 'softer' cams that make an engine last longer and be more reliable.

From cams to valves...
Valves allow the mixture to enter and leave the engine at specific times. They are subject to certain constraints. One is the physical size of the valve and another is how far they open at full lift. They will come very close to the piston crown in a tuned engine so care must be taken when trying modifications with valve lift and valve sizes. If the valve touches the piston at hand turned speed just imagine the mess at 6000 rpm when everything ‘grows’ as all the clearances in the engine add up.

You can of course check any close clearances by using a squashable medium such as plastigauge, or even plasticine if that is all you have. Place the compressible medium at the possible trouble spot and rotate the engine in the direction of use and then lift the head to see what thickness the compressible medium is now reduced to. If it is sufficient you are ok to try the engine at normal speed, but if it is compressed to a few thousands of an inch then you have a problem and need to alter the clearance to create space for the moving parts to work without collision.

We can now look at the options for valve control. The basic method of opening and shutting a valve is to open it with a cam, which pushes a cam follower, which houses a pushrod, which transmits force to a pivoting rocker arm, which pushes down on the valve stem and 'opens' the valve, by lifting it clear of the valve seat and allowing ingress or egress of fuel/air mixture or exhaust gases. The valve stays open as long as the cam allows and closes as the cam follower moves down the ramp of the cam and allows a valve spring to return the valve to its seat and hopefully seal this area during the next stroke.

Alternatives to spring closing valves can be... Why do we have valves anyway?
The four stroke engine uses valves to breathe in and out. The piston is just a lump of metal moving up and down the cylinder at the beck and call of the crankshaft.

Lets look at the piston...
The piston uses piston rings, made of springy steel to contain the gases within the combustion chamber during the compression cycle and during the power cycle. Without the piston rings, or with worn rings, the gases produced by compression and ignition would just pass the piston and end up in the crankcase. Very little power would be produced and the engine would be disappointing in terms of performance. In short we need those rings to enjoy our racing! Ok so the rings might be great but the cylinder bore might be worn or glazed. The effect will be that of poor performance again... This is getting to be a habit! What should we look for and more to the point what can we do about this poor performance lark?

Well we can... The piston is attached to the connecting rod with a case hardened tubular piece of metal called the gudgeon pin. Contrary to common opinion I would suggest that this is a plain bearing and as such needs constant lubrication to perform as designed. The gudgeon pin can and does wear eventually. The wear shows as dull markings in the shiny areas of the bearing. The gudgeon pin will be held central in the piston by the use of circlips. Always take care not to bend these to the point that they do not snap into their respective grooves when fitted. The gudgeon pin should be a neat push fit into the piston, If too tight or slack problems will occur. Look for burrs at the entrance to the piston if the gudgeon pin is tight.

The Con rod is there for a reason...

The con rod is a misunderstood member of the engine club. It is oddly shaped and seems to be there just to make up the numbers. Not so! The con rod is the part that keeps everything else in line. It converts rotary crankshaft motion into almost linear motion as it pushes and pulls the piston up and down inside the cylinder. The con rod has a big end, which surrounds the crank pin of the crankshaft and a little end to support the gudgeon pin, which in turns supports the piston. The two are kept apart by a H section casting. This is the area tuners look to when saving weight. Some big end bearings are little more than white metal coatings. The white metal needs copious supplies of oil to avoid wearing out. True competition engines use needle roller bearings at the little and big ends. These take the loads of racing far easier than a dab of white metal and oil levels that are not reliable and use splash theory to lubricate the internals.

The crankshaft is not straight...

The most common engines have a cast steel crankshaft, so called because at its centre is an offset crank pin which, when the crankshaft is rotated, moves in an exaggerated rotation, called the stroke, to convert linear forces to rotary motion, and is supported by bearings at each end. Lots of weight saving methods are done in this area. The effect is to lighten the whole reciprocating assembly and allow the engine to accelerate quicker. How much weight can we take off? This subject, called the balance factor, is too broad for this basic tech talk but is well worth the effort when the resulting engine spins so much faster and vibrates less.


The need for the carburettor is to accurately meter the fuel/air mixture for combustion as near as perfect as possible. It serves to hold enough fuel to feed the engine under fierce acceleration and during high cornering forces. In principle it is probably wise to fit a massive carburettor but in practice too large a carb will just bog the engine down under acceleration. The happy medium is a carb that provides loads of fuel/air mixture and also accelerates quickly from slow corners or a standstill. To establish this we need to consider the colour of the plug when removed from an engine that has just done a long fast workout. The optimum colour is light brown. Black is too rich and indicates too much fuel present. White means too weak a mixture and probably makes the engine feel as if it is about to expire due to lack of breath. Weak mixture can result in valve ends dropping off, piston seizure, due to extreme heat in the combustion chamber and poor performance. Too rich shows as lumpy acceleration, intermittent power and poor top end performance. Take time to let the engine breathe easily, jet the engine for the conditions and you will reap the benefits of an engine that performs well.

The exhaust system...

Lets just run with no exhaust and enjoy the 'racing like' noise... NO NO NO. This is not the way to go fast and could well lead to a fire or worse. The perfect exhaust would be a constantly tapering tube directly leaving the exhaust port and carrying on straight to a megaphone like end pipe. Unfortunately racing karts have neither the space or the classes to allow for this 'perfect' system. A compromise is a bent system of sufficient taper to extract the exhaust gases when required and still keep within the acceptable noise levels of racing today.

Igniting the blue touch paper...

The touch paper is in fact a spark plug and it is usually only blue at the end when sparking. The ignition system of a common four stroke operates at very high voltages. The high voltage is produced by an ignition coil. The point at which the spark occurs, during the compression stroke, is called the ignition timing. Making the spark occur earlier is called advancing the timing and conversely allowing the timing to occur later than normal is a called retarding the ignition.

General principles of advance and retarding the ignition: Hopefully this covers the basics of four stroke theory and gives you the reader a taste for engineering and a need to delve deeper. Tuning a four stroke is easy, find a good manual, try it and enjoy yourself.

This article was produced by Brian Pollard, author of "Preparing the Gx160 for 'open' racing" which is available on CDROM, in multimedia format, as an e-book, and in paper form. All enquires should be sent to the above e-mail address.

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