Basic diesel engine components

4-stroke and 2-stroke diesel engines both use the principles of internal combustion, so many of their components have similar designs.
Diesel engine components are exposed to higher operating temperatures, pressures and forces than gasoline engines of similar size. Their compression ratios are higher, and they are often designed to out-last gasoline engines. Their engine parts are usually heavier or more rugged than those of similar output gasoline engines.
Diesel blocks are usually made of cast iron, and heavier than in a gasoline engine. The skirt of the block usually extends below the centreline of the crankshaft. This adds strength and rigidity.
Machined into it are the cylinders which are usually in the form of detachable sleeves or liners.
It is sealed at one end by a deep-section piece of metal or alloy called a cylinder head, which houses the valves and injectors.
Most cylinder heads in diesel engines are cast iron. Depending on the engine design, single or multiple heads can be used.
Multiple heads avoid large castings that, apart from being heavy, are liable to distortion.

Diesel engine passages

Both 4-stroke and 2-stroke diesel engines have passages cast in the head to carry oil for lubrication and water for cooling.
The combustion chamber can be formed in the cylinder head or the piston crown. These chambers are different from those in gasoline engines. That’s because diesel fuel is different from gasoline and so is the way it is ignited.
In a gasoline engine, fuel already mixed with air enters the cylinder, and a spark plug ignites it. That’s why gasoline engines are called spark-ignition engines.
In a diesel engine, just air enters the combustion chamber first. It is then highly compressed, and its temperature rises. Fuel is injected. It ignites, due to heat of the compressed air. That’s why diesels are called compression-ignition engines.
Injectors are mounted in the cylinder head so that they reach into the combustion chamber.
They inject fuel into the chamber in atomised form - in a fine spray.
Atomised fuel burns more efficiently than liquid fuel.
Different spray patterns are used, depending on the shape of the combustion chamber.

Diesel fuel delivery

Different diesel engines use different fuels.
Fuel can be delivered to the chamber in different ways.
Direct injection means fuel is injected directly into the chamber. The cylinder head usually has a flat surface and the combustion chamber is formed in the piston crown. At top dead centre, there is very little clearance between the cylinder head and the top of the piston.
Another method of injecting fuel is Indirect injection.
Fuel is sprayed into a smaller, separate chamber in the cylinder head.
This chamber can have various designs.
A glow plug is not a spark plug. It is a small electric heater that pre-heats the separate chamber as an aid to cold starting. It helps the combustion which then spreads to the main chamber.

Direct injection

Direct injection is usually used for larger diesel engines and 2-stroke diesel engines, while indirect injection is usually used for smaller 4-stroke diesel engines.
The way 2-strokes and 4-strokes get air is also very different. In a 4-stroke diesel, just as in a gasoline engine, the inlet and exhaust ports are controlled by valves. But the much higher operating pressures and temperatures in diesel engines put more stress on diesel valves which are usually larger than those in gasoline engines. The intake valve passes only air so it is cooler than the exhaust valve which releases all the hot gases after combustion.
Valves in the diesel engine are usually parallel to the centre-line of the engine.
Small 4-stroke engines usually have 2 valves per cylinder. 1 inlet and 1 exhaust.
This 2-stroke diesel engine lets air into the cylinder through inlet ports with no inlet valve. But its exhaust ports do have valves.
4-strokes and 2-strokes use the same system to control their valves.

Diesel valves & components

The valve is held in place by a valve guide, with a spring on its stem.
A rocker arm compresses and releases the valve spring, so that the valve opens and closes.
They don’t operate at random. They need a system to control them. Part of that system is the cam.
A Cam is a lobe, on a shaft. It is specially shaped to open the valve, hold it open, and let it close.
So the cams control the valve action, but what drives the cams?
Cams are attached to a camshaft.
In many diesels, valves are driven by a pushrod system.
The camshaft is mounted in the engine block near the crankshaft, and gear driven by the crankshaft.
As the camshaft rotates, a cam bears against a cam follower that pushes up a pushrod. The rocker arm pivots and opens the valve.
some small high-speed diesel engines uses a camshaft in the cylinder head. This is called an overhead camshaft and it is usually driven from the crankshaft.
In most 2-stroke diesel engines, the camshaft operates the exhaust valves through rocker arms, and they usually have 1 camshaft for each bank of cylinders.
In both systems, the camshaft keeps the valves working in sequence.
In the 4-stroke, valves must open only when the piston is in the correct position.

Diesel scavenging

In a 2-stroke diesel engine, an air pump or blower provides clean incoming air at pressure slightly above that of outgoing exhaust gases.
The blower delivers air to an air-box surrounding the cylinder.
When the piston uncovers the inlet ports, air enters the cylinder through a number of them around the cylinder walls.
This fills the cylinder with fresh air and helps blow out exhaust gases. Removing exhaust gases is called scavenging. Without effective scavenging, diesel engines lose efficiency.
How scavenging occurs depends on the design of the cylinder and ports. In this example, the air flows in one direction, towards the top of the cylinder. This is called uniflow scavenging.
Crossflow scavenging occurs when air enters on one side of the cylinder, and exhaust gases exit on the other side. No valves are used, and the airflow is across the cylinder.
In loop scavenging, air enters across the top of the piston and flows up in a loop before moving in the direction of the exhaust port.

Crankshaft rotation

The position of the piston is just as crucial in this 2-stroke diesel engine. Its exhaust valve must open and close at the right point in the engine cycle.
In one 2-stroke engine cycle, the crankshaft makes 1 revolution. How does this compare with a 4-stroke engine cycle? How many revolutions does its crankshaft complete from Intake, through Compression, Ignition, and Power, to Exhaust? It makes 2 revolutions. In that time, what happens to the valves? They open and close once. So the cams must make just 1 revolution. And if the cams make 1 revolution, so must the camshaft.
But in each 4-stroke cycle, the crankshaft rotates completely twice. So the camshaft must rotate at half the speed of the crankshaft. This can be achieved by gears.
Take 2 gears with teeth the same size - one with 40 teeth and the other, the driving gear, with 20. When they mesh, the larger one rotates at half the speed of the smaller one. The gear with twice as many teeth turns at half the speed of the other.
These gears turn in opposite directions and the one with fewer teeth rotates faster. Now let them be separated but linked by a chain. They turn in the same direction, at the same speeds as when they were meshed.
If the smaller gear is attached to the crankshaft and the other to the camshaft, how fast will the camshaft turn compared with the crankshaft? It will turn at half the speed. Just what the system needs.
Everything happens at the right time. The piston leaves top dead centre, the crankshaft turns, and the camshaft turns too. The cam acts on the rocker arm. It opens the valve - as required.
This is a 2-stroke diesel engine. How many revolutions does the crankshaft make in one cycle, from Intake, Compression, Ignition, Power to Exhaust? It makes 1 revolution. During that revolution, the valve opens and closes once. The camshaft makes one revolution too.
Camshaft and crankshaft must rotate at the same speed.
The gears must be the same size.
The power stroke pushes the piston down. The crankshaft turns, as does the camshaft. The cam acts on the rocker arm, which opens the valve - at the right time.

Diesel crankshaft

The crankshaft is the same on 4-stroke diesel engines and 2-stroke diesel engines. The shape may appear unusual at first.
Why won’t a simpler shape do the job? It’s all to do with balancing forces.
This top is evenly balanced, and it spins smoothly. Adding a weight unbalances it. How can it be made to spin properly again? One way is to put an identical weight on the other side. The first weight is balanced by the second weight. It acts as a counterweight.
Counterweights turn up in unusual places. Some systems can’t work without them.
Crankshaft counterweights keep the rotating components in balance and help the crankshaft turn as smoothly as possible.
The crankshaft turns because of the forces transmitted through the connecting rods. It must also be held in place. That’s done by bearings.
Different kinds of bearings have different designs. They reduce friction, and allow free movement.
The Crankshaft is held in the engine block by main bearings at points called journals. The crankshaft also needs to be located to stop lateral movement. This is done here by using flanges.
Between the connecting rod and the crankshaft are connecting rod bearings. They protect the spinning crankshaft at points called journals.
On the rear of the crankshaft of both the 4-stroke & 2-stroke diesel is a heavy flywheel.
It stores energy from the power stroke and gives it to the crankshaft to help it keep turning. In a 4-stroke, only 1 stroke in 4 delivers power. The energy from this 1 power stroke has to turn the crankshaft through the other 3 strokes. Without a flywheel the crankshaft would slow down and stop.

Pistons in 2 & 4-stroke diesel engines can change direction hundreds of times a second and are exposed to extremes of heat and pressure. Modern pistons are made of aluminium alloys.
Diesels using direct injection have an almost flat surface on the cylinder head and almost all of the combustion chamber is in the head of the piston.
Engines with indirect injection usually have pistons with a flatter head, and sometimes with small indentations.
When the piston is fitted, there must not be too much clearance. It has to seal in the high pressures and temperatures generated by combustion. This is done by piston rings - held in grooves in the side of the piston.
The top two are called compression rings. The lower ring is an oil control ring. It scrapes excess oil off the lower cylinder walls.

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Monday, December 6, 2010

Motive Power Types : Compression-ignition engine components