- The lubrication system
- Splash system
- Pressure system
- 2-stroke engine premix fuel systems
- 2-stroke engine oil injection systems
- Rotary engine lubrication system
- Corrosion/noise reduction
The lubrication system
All moving parts in an engine require lubrication, that is, the application of oil to moving parts to reduce friction between them.
A lubrication system distributes oil through the engine. A typical lubrication system consists of an oil sump, an oil pump, and oil galleries. The oil is stored in the sump, bolted to the bottom of the engine block. Oil is drawn from the sump by an oil pump. Oil galleries are small passages in the cylinder block that direct oil to the moving parts.
Oil that has been pumped to the crankshaft main bearings travels through oil-ways to the connecting rods.
Oil may also be splashed from the connecting rods onto the cylinder walls.
A filter is usually provided to remove particles of dirt from the oil, and the circulation of the oil assists with the cooling of the internals parts.
Splash system
Most small 4-stroke gasoline engines use what is called splash lubrication.
On horizontal-crankshaft engines, a dipper on the bottom of the connecting rod scoops up oil from the crankcase for the bearings. In this engine it is also able to splash oil up to the valve mechanism. Alternatively, an oil slinger can be driven by the camshaft.
A similar system is used in most small vertical-crankshaft engines. Oil is also splashed up to the valve mechanism.
Pressure system
Modern vehicle engines use a pressure or force-feed lubrication system where the oil is forced around the engine under pressure. Let’s look at gasoline engines first.
Oil won’t flow up into the engine by itself so a pump collects it through a pickup tube and a strainer, and forces it through an oil filter, then into passageways in the engine block, called galleries.
The galleries allow oil to be fed to the camshaft bearings, the valve mechanism, and the crankshaft main bearings. Holes drilled in the crankshaft webs allow the main bearings to supply oil to the big-end bearings.
After circulating through the engine, the oil falls back to the sump to cool. This is called a wet-sump lubrication system because the oil is kept in the sump ready for the next time it’s used.
Some special engines use a dry sump lubrication system. It uses all of the parts that make up a wet sump system and it lubricates the engine in the same way.
It differs from the wet sump system in the way the oil circulates. In a dry sump system, the oil falls to the bottom of the engine into an oil collection pan. A scavenge pump then pumps it to an oil tank where it is stored until the normal oil pump collects it and pumps it through the filter and engine in the normal way.
Because there is no oil storage sump under the engine, the engine can be mounted much lower than in a wet sump system.
The oil tank can be positioned away from the engine where it can get best cooling. And the amount of oil in the system can be much greater than in the wet sump system.
Diesel engines are lubricated in much the same way as gasoline engines but there are differences.
Diesel engines typically operate at the top end of their power range so their operating temperatures are usually higher than those in similar gasoline engines, so the parts in diesel engines are usually more stressed.
As a result, diesel oils need a different range of properties and are classified differently.
It’s common for some diesel engines to use an oil cooler to cool the oil in the engine. The cooler and oil filter are usually on the same mounting on the cylinder block.
2-stroke engine premix fuel systems
Most 2-stroke gasoline engines use a set petrol-oil mixture for lubrication.
There are different kinds of oils and petrol-oil mixtures. They must be used according to the manufacturer’s specifications.
For small 2-stroke engines, the oil and fuel are pre-mixed.
A mixture of air, oil and fuel passes through a sealed crankcase on its way to the combustion chamber. That makes the crankcase part of the fuel intake system so it can’t be used as a sump.
As the air, fuel and oil enter the crankcase, the fuel evaporates, leaving behind enough oil to keep parts coated and lubricated.
The crankshaft and connecting rod bearings in 2-stroke engines are ball and roller types. Because the parts of this kind of bearing only roll over each other they only need a minimum of lubrication.
2-stroke engine oil injection systems
Some 2-stroke gasoline engines use an oil injection system.
This system doesn’t need the oil and gasoline mixed manually. An engine-driven oil pump takes oil from a tank and pumps a measured amount directly into the engine where it will mix with the fuel and lubricate the internal engine parts. The pump delivers the correct amount of oil for the engine speed and the throttle setting, which makes it a more precise system than running the engine on a fixed oil-gasoline mixture.
Rotary engine lubrication system
The main lubrication system of a rotary engine is similar to that of a 4-stroke gasoline engine.
An oil pump picks up oil from the oil pan through a strainer, and delivers it to an oil cooler on the front of the engine. As the oil cools, it then enters the oil filter at the rear of the engine. A regulator valve controls the oil pressure in the system. The oil goes on to the main bearings and rotor bearings, and is sprayed into rotor cavities. Along the way, it absorbs heat, then returns to the oil pan.
A bypass valve at the end of the eccentric shaft helps the engine warm-up.
When the engine is cold, it stays open and sends oil from the shaft to the oil pan, and stops it spraying on the rotors.
As the engine heats, the valve starts to close. Pressure in the shaft builds up, and cooling oil starts to spray on the rotors.
The rotary engine uses oil injection. A pump sends oil to oil nozzles in the intake manifold, and to oil nozzles in the intake passages in the rotor housings. Oil from these nozzles goes to the engine and lubricates the rotor seals.
Corrosion/noise reduction
Corrosion/noise reduction in the lubrication system
Engine oil performs many other functions apart from lubricating moving components. Two other functions are corrosion protection and noise suppression.
Corrosion protection
Acids build up in the engine due to the accumulation of combustion by-products and moisture.
Blow-by gasses contain chemicals that are trapped in the oil. The chemicals react and form acids.
When the engine is turned off it begins to cool. As it cools, fresh air that contains small amounts of moisture is drawn into the crankcase via the PCV system. The moisture condenses into droplets that fall into the oil and form acids.
The acids attack the internal components causing unnecessary damage. The oil contains anti-corrosion additives that coat the engine surfaces, protecting them from the effects of the acid.
Noise suppression
As oil is pumped through the engine it is forced between moving components. This protects the components from wear and acts as a shock absorber dampening the shock loads between those parts and quietening the engine operation. The oil also coats all the internal walls of the engine providing an insulating barrier to noise.
