Basic Principles Of Diesel Engine

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The Direct Injection Engine (DI)

The engine uses the heat caused by compressing air in a cylinder, with a piston, to ignite fuel oil which is injected into the cylinder. This oil burns and creates an increase in pressure which forces the piston back down the cylinder, providing the power. The burnt gasses are exhausted from the cylinder and replaced by fresh charge through valves, commonly positioned above the piston in the cylinder head.

The fuel oil is pushed into the engine under high pressure at the correct time by an injector pump. The oil is sprayed into the engine cylinder as a very fine mist from the nozzle of an injector. The fine fuel oil mist readily mixes with hot air in the cylinder, ignites and provides an efficient burn. The injector pump also meters the amount of fuel delivered. The more fuel the more power/faster the engine.

The Indirect Injection Engine (IDI)

A later development was the IDI engine that utilizes a separate combustion chamber, connected to the engine cylinder, into which the fuel is injected and combustion is initiated.

A heat resistant insert with low heat conductivity is located within the combustion chamber so it quickly heats up and retains heat from combustion, providing extra heat to enable quicker ignition. The fuel is injected into the hot combustion chamber as a jet at a low pressure compared to the fine high-pressure spray of a DI engine. The fuel jet hits the hot insert where ignition is initiated; the fuel is distributed around the combustion chamber as combustion continues. The expanding burning fuel, along with partially burnt and unburnt fuel, is carried into the hot engine cylinder where further oxygen is available and combustion Continues.

The most common prechamber format utilized is the Ricardo Comet design developed by Ricardo and Company of Shoreham, Sussex, UK. With this design, air is pushed from the cylinder into a circular ‘swirl chamber’ through a tangentially aligned throat. The bottom half of the chamber along with the throat is constructed from a nimonic alloy designed to maintain high temperatures during engine operation. The temperature of the compressed air is raised further while passing through the throat. A vigorous swirl motion is initiated as the air is forced into the circular swirl chamber. The fuel is injected into the swirl chamber and rapidly atomized within the mass of hot turbulent air.

The advantage of IDI engines is that they can operate at higher engine speeds as the more efficient fuel and air mixing provides faster combustion. Cars and small commercial vehicles require a small, light engine which must be able to operate at higher speeds to provide the necessary power, and with the advent of the IDI engine the use of diesel engines in such vehicles became widespread.

The heat lost due to the increased surface areas of the combustion chamber and the pressure drop between cylinder and combustion chamber make it necessary for the engines to operate at higher compression ratios to provide enough heat for ignition. The lost heat and force required to push the air into the combustion chamber is wasted energy making IDI engines around 10-15% less efficient than DI units.

IDI engines became the engine of choice in small vehicle applications as a small engine could produce more power at higher speed providing a suitable power/weight ratio for such applications. Recent advances in fuel injection technology, which provide more precise fuel delivery, allow faster combustion within a DI engine. The improved efficiency of the DI cycle has spurred the fitment of such engines to become more common in small vehicles.

Source: Darren Hill, “Vegetable Oil as a Fuel”

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