Early forms of the internal combustion engine were pretty rubbish. Mimicking the workings of steam power, gases were allowed to ignite of their own accord without any of the necessary ‘squeeze’, which proved hopelessly inefficient.
Give or take a few decades later and what’s now become the familiar four-cycle combustion process was largely perfected with motor cars from the likes of Daimler and Benz being able to move under their own (excuse the pun) steam shortly afterwards.
In case you’re hazy about the exact order of events every time you turn the key of your four-stroke classic, here’s what happens:
On the intake stroke, the piston moves downwards sucking a predetermined vapourised mix of air and fuel via the inlet valve as a result of the camshaft lobe pressing down on the valve stem. The inlet valve closes before the piston comes thumping back up for the compression stroke, at which point both valves are closed and the fuel/air mix increases in pressure, temperature and density. Then, boom! When the piston reaches a point just before top dead centre (TDC) the spark plug ignites the mixture (power stroke) and the piston is shoved downwards again by the expansion of the gases and this movement is translated to the crankshaft. On the exhaust stroke, the exhaust valve opens, the piston moves back up and the spent exhaust gases are spat out. The sequence then starts again, with the process repeating every two revolutions of the engine.
Inevitably there’s some blow-by via the piston rings – and positive crankcase ventilation (PCV) systems are used to redirect some of this unburnt mixture back into the inlet tract to avoid nasty emissions to the atmosphere.
So those are the fundamentals, but perhaps what’s more interesting is how engineers have meddled to make the whole affair more efficient. The most obvious tweak has been via turbocharging, whereby exiting exhaust gases are harnessed to force in more air and fuel, raising the mixture’s density thus aiding the combustion process. As a measure of its success, virtually every new car made today has one fitted. A supercharger works in the same way, of course, but is driven mechanically via a pulley rather than by exhaust gases.
Key, obviously, to further refinement of the combustion cycle has been to find ways of burning all of the mixture as completely as possible. Boffins have modified both the shape of the combustion chamber, the design of the piston head and added more valves per cylinder, as well as getting the mixture to ‘swirl’ in just the right way so it burns more quickly and efficiently.
The combustion ratio comes into play here too. In a nutshell, this is the ratio between the volume of the cylinder and the combustion chamber when the piston is at the bottom of its stroke, and the combustion chamber when the piston is at the top of its stroke. Higher compression ratios are desirable, of course, because more mechanical energy can be extracted from a given air/fuel mixture due to its higher thermal efficiency. In other words, higher-compression ratio engines allow the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lower exhaust temperatures. That’s all good stuff because basically any heat represents wasted energy. But with high ratios and low octane fuel there is the risk of engine knock where one or more pockets of air/fuel explode when they shouldn’t, which can damage the engine as cylinder pressures increase.
A way round this, and one that’s being increasingly adopted by manufacturers to increase engine performance and economy, is the use of direct fuel injection – this injects fuel at high pressure via a common fuel rail directly into the combustion chamber. The fuel is regulated to correspond to different engine loads, making throttle response and engine speed management as precise as possible.
From an aftermarket tuning point of view, there’s also a raft of modifications that can be made to help the whole internal combustion cycle spin a lot more smoothly. High-lift camshafts, for instance, increase the duration that the valve is off its seat – and as a rule, the greater the duration, the greater the horsepower. However, increasing duration comes at the sacrifice of driveability – peak power occurring much later in the rev range with a loss of low-down grunt.
Bigger valves, polishing and porting to aid efficient air flow in and out of the combustion chamber also helps, as does (of course) fitting a bigger bore exhaust to allow spent gases to escape more quickly. The possibilities are endless.
As for the future, who knows where technology will take us. Hydrogen fuel cells, steam even? One thing’s for sure – we’ll be enjoying the more than a century old suck, squeeze, bang, blow process in one form or another for many more years to come.
