PC ACADEMY
Back to basics in the classroom
The most prolific prime mover, we explain the theory
OVERVIEW
The four-stroke (Otto) -cycle, engine converts chemical energy (hydrocarbon fuel) into kinetic (movement) energy in order to do useful work. It is essentially an air pump, using heat to selectively expand gas to force a piston(s) down a cylinder(s) and is therefore an internal combustion engine (ICE), unlike steam engines whose combustion occurs externally (ECE) to the piston/cylinder.
The most common fuel used is petrol, requiring a spark to ignite its vapour to create heat (spark ignition or SI) followed by diesel fuel, which uses compression to ignite the fuel (compression ignition or CI). Other fuels used are liquefied petroleum gas (LPG), alcohol, paraffin, vegetable oil, etc.
Having undergone many refinements, it continues to be the engine-type of choice since its inception in the late nineteenth century despite only converting about a third (petrol) to a half (diesel) of the fuel into useful energy, depending on whose research you heed. As the most widespread in classic car usage, we’ll feature an SI overhead valve (OHV) engine in this article. In reality, most engines have multiple cylinders, commonly four, producing four power-strokes for every two crankshaft revolutions.
2 Volumetric efficiency
Effectively filling the cylinder with air-fuel mixture is called volumetric efficiency and has been a battle for designers.
There are many factors affecting volumetric efficiency, such as valve size and timing, shape and design of inlet and exhaust ports/tracts, valve timing, and atmospheric pressure, among many. Supercharging with an exhaust-gas-driven turbocharger or engine-driven supercharger largely overcomes this.
START
1 Induction stroke
The inlet valve opens just before the piston starts to descend on the induction stroke. Atmospheric pressure fills the cylinder with air only if it’s a diesel or direct-injection petrol engine, or air-fuel mixture if carburetted/fuel injected. If a supercharger or turbocharger is fitted the inlet charge is pressurised.
8 Exhaust stroke
The exhaust valve opens before the piston reaches bottom-dead-centre on the power stroke, much of the energy having been extracted from the expanding gases. The gases exit via the exhaust port, the rising piston clearing them before beginning another stroke.
The exhaust valve can reach temperatures in excess of 700°C.
7 Component Parts
A typical overhead-valve four-stroke engine like this one comprises the main rotating components: the crankshaft A, connecting rod B, piston C, camshaft D and cam followers E, cam and crankshaft timing gears F, all housed in the cylinder block G. At the top end is the inlet valve H, exhaust valve I, combustion chamber J, rockers K, valve springs L, pushrods M, and cylinder head N.
TOP TIP
The crankshaft turns a further 180°, completing two turns or 720
