The four stroke cycle is commonplace in many internal combustion engines today. Cars, motorbikes and even aeroplanes all use a four stroke cycle. This cycle is made up of four reciprocating movements (or strokes) of a piston in a cylinder. Both petrol and diesel engines feature these cycles, but petrol uses a spark plug for ignition, while diesel relies on the heat and increased pressure of the compression.
The strokes are as follows:
- Intake (or Suction) Stroke
- Compression Stroke
- Power Stroke
- Exhaust (or Expulsion Stroke)
The process basically consists of - as German engineer, Nicolaus Otto, illustrated - adiabatic (a thermodynamic process in which no heat is transferred) compression, heat addition at a constant volume, adiabatic expansion and then heat rejection. The power derived from this process comes from the expansion of gases in the power stroke. The fuel is injected during the compression stroke and then, whilst combined with pure air accumulated during the intake stroke, is compressed, ignited and then released, pushing the piston back down and creating energy. Not all of this energy is used though; some of it - roughly 50% in a four stroke engine - is rejected as heat through the exhaust and cooling systems in the exhaust stroke. This is still more economic than a two stroke engine though, which wastes roughly 60% of energy.
Intake (or Suction) Stroke
The intake stroke is the first in the four stroke cycle; it begins with the piston at top dead centre (the point when the piston is furthest away from the crankshaft). It moves down the cylinder as the intake camshaft opens the intake valve. This creates a partial vacuum and draws pure air in. The exhaust gases that leave the cylinder during the exhaust stroke of the preceding cycle reduce the pressure inside and make it easier for a vacuum to form. This works in the same way you create a partial vacuum with your mouth on a drinking straw: the pure air rushes into the cylinder to fill the space; the water climbs up the straw.
Compression Stroke
Once the pure air has been sucked into the cylinder, and the piston is at bottom dead centre (the point where it is nearest the crankshaft and connecting rod), the second stroke - the compression stroke - can begin. The intake valve is closed and the piston moves up the cylinder, compressing the air inside. This compression causes the pressure and the temperature inside to heat up drastically. Diesel engines are much more economical than petrol engines during this stage. In a petrol engine, the volume of air when the piston is at bottom dead centre is compressed to about an eighth of its original state. In a diesel, it can be compressed closer to a sixteenth. This, coupled with the fuel that is injected into the cylinder towards the end of the compression stroke, creates a highly combustible mixture.
Fuel Injection
Liquid-state fuel is injected into the cylinder and precisely the right point and rate to be ignited by the compressed pure air created by the compression stroke. The fuel is vapourised by the heat, producing energy via the power stroke.
Power Stroke
The power stroke begins at top dead centre and is the point in the cycle that produces the force that powers our cars and machines. The ignited fuel quickly expands in all directions inside the cylinder. Since it has nowhere else to go, it pushes the piston down towards the crankshaft at speed. This energy follows suit through the connecting rod and crankpin, and gives the crankshaft a powerful burst of energy. This force is used to turn the wheels on a car - the result of the four stroke cycle's generation of power.
Exhaust Stroke
The gases left inside the cylinder must be cleared once the power stroke is complete. The piston, now at bottom dead centre, starts its return to the top, where the cycle began. The exhaust valve is opened by the exhaust camshaft, and the excess burned gases are pushed out of the cylinder. Now, the process can start all over again. This four stroke cycle will be repeated continuously whilst an engine is running and can perform over 100 cycles a second.
The Differences Between Petrol & Diesel Four Stroke Engines
The main difference in the way petrol and diesel four stroke engines work is in the amount of fuel necessary. Because diesel engines have a better compression ratio, they can depend more on the temperature and pressure created by compressing pure air to ignite the fuel. Petrol engines, on the other hand, have a lower compression ratio and so require more fuel to burn. This is achieved by pushing fuel into the cylinder at the same time air is being drawn in. The fuel-air concoction is then compressed and ignited by a spark plug. Diesels are therefore more economical with fuel.
Compression Ratio
Compression ratio is calculated by measuring how much the volume of the pure air inside the cylinder is compressed by. For example, if there were a volume of 100 cubic centimetres compressed to the volume of 10, it would have a 10:1 ratio. This is because it has been reduced to 10 per cent of its original state.
Risks
If the fuel is not ignited at the appropriate time or rate, the engine can be damaged. Fuel burnt before the compression stroke has finished pushing the piston to the top of the cylinder could push the piston back down against its rotation. This is a result of highly flammable fuel with a low self-ignition temperature being used.
Engine knocking is also a risk. This is where an unintended separate ignition occurs within the cylinder in addition to the one that is intended to produce the power stroke. There is a period of lag between when the fuel is injected into the compressed pure air and when it is ignited. This can cause a surge beyond the design's capability and consequently damage or overwork the system.
Benefits of a Four Stroke Engine
Aside from the already established economical advantages, there are a number of advantages four stroke engines have over petrol four strokes, and two stroke engines:
- Longer engine life than two stroke engines - four stroke engines have a lubricating system
- Wastes less energy, and consequently pollutes less
- Better mileage - two stroke and petrol engines require more fuel
- No need for specialised oil to maintain it - two stroke engines run on a particular oil, premix, which adds to the cost to run
Four stroke engines have grown considerably since their conception in the mid 1800s. Many engineers have contributed to their rise. The first diesel engine using a four stroke cycle was produced in 1897, sometime after the four stroke petrol engine had been developed. Today, use of the four stroke engine is extremely widespread; it now an integral component in the manufacturing of automobiles.