4 stroke diesel engines are fundamentally different to the petrol counterpart, for a start there is no ignition system or carburettor!
So what are the similarities - well there are inlet and exhaust valves and the camshaft to operate them!
Lets start with the fuel, it's basically a thin oil, and yes the very first diesel ran on peanut oil. In order to get the oil to burn you need high pressures and temperatures - to get both you simply compress the air charge to a high degree and then inject the fuel. Diesel fuel actually contains more energy than petrol for the same volume or weight.
So how does it all work - well it's the usual 4 stroke SUCK - SQUASH - BANG - BLOW.
There is no throttling of the air supply on a diesel, even at idle the motor receives a full charge of air, all the power control is achieved by controlling the fuel supply.
So sucking in a full air charge, this is then compressed by the rising piston to about a twentieth of it's original volume, this raises the temperature too. Ever pumped up a bicycle tyre and held on to the bottom of the pump - yes it gets very hot. The top of the piston has a little bowl in the top and this is where the hot compressed air is forced into.
Queue the injector - this squirts a measured amount of fuel into the piston bowl which immediately burns (it does not explode) as it does gasses produced expand rapidly delivering the power and pushing the piston down, as it approaches the bottom of the bore the exhaust valve opens and out go the spent gasses. The heavy flywheel keeps the crank revolving and the cycle repeats until it either runs out of fuel or air (or something breaks).
In very simple engines such as the Greaves fitted to my bike are simplicity itself it utilises a single cam lobe to operate both valves and the injector pump too.
Diesel Engine Piston Crown
Cylinder Head - that's the injector tip not a spark plug!
How do you get the fuel in?
In order to inject the fuel into the cylinder at the right time you need 3 components, a regulator, a pump and an injector.
The regulator controls the amount of fuel that the pump delivers, it does this based on 3 controls, the engine speed, the 'request for revolutions' (throttle) and the control settings.
The pump compresses a set amount of fuel as requested by the regulator and pushes this through to the injector.
The injector passes the fuel into the combustion chamber.
Regulators have 3 pre-settings - the idle speed, the max speed and the maximum fuel volume.
The first two are easily understood the last - max fuel volume is the largest fuel dose that the engine can handle, set this too low and power loss is the result as there will be leftover oxygen after the combustion is completed. Set it too high and watch the smoke produced - this is unburned fuel and will raise combustion chamber temperatures sharply - a seizure usually follows if left uncorrected.
Internally there will be a centrifugal control adding to the mix as well, all these elements combine to control the volume of fuel that the pump delivers.
This is a high pressure engine driven piston type pump, on simple engines such as the Greaves this is driven by the same camshaft lobe as the valves. It is timed to deliver the pump stroke at slightly before Top Dead Centre.
It is connected to the injector by a steel pipe capable of withstanding the 3000 psi that the pump delivers.
The Injector is the simplest part of the whole system, at the delivery tip are some tiny holes through which the fuel is delivered to the combustion chamber. It is manufactured from some fairly exotic materials capable if withstanding the combustion process. Internally it has ports and a control spring that together act as a cascade that allows the fuel to pass through once a set pressure is reached.
There is a fuel return line that allows excess fuel to be returned to the supply tank, this prevents the system from becoming hydraulically 'locked' as fuel seeps into the spring chamber.