Exhaust Gas Recirculation

Exhaust Gas Recirculation

(EGR) System Diesel engine always operate with excess air (or weak

mixture) and with high

exhaust gas temperatures, this oxygen combines with Nitrogen to form the

pollutant NO X (Oxides of

Nitrogen). Exhaust gas recirculation (EGR) is often used to reduce cylinder temperatures and to reduce the amount of oxygen in the cylinder air charge, thus preventing the

formation of NO X .

Combustion Phases For diesel engines, there are three main phases in the combustion process. The first is the delay phase as the fuel absorbs heat from the cylinder air charge and vaporises. The next phase Phases of combustion occurs when the fuel has reached a sufficient temperature to self-ignite, this causes combustion and the flame front propagates rapidly out across the piston crown. This is where the rapid energy release occurs and causes the characteristic diesel engine noise. Once initial burning of the fuel takes place, continued injection of fuel provides a controlled burning and energy release to provide sustained pressure on the piston and good torque generation at the crankshaft.

Good Combustion Efficient and effective combustion promotes an engine with good power output and with minimal harmful emissions. This can only be achieved when the engine mechanical parts are in good

Good… Combu stion

condition and, the engine control systems for fuel delivery, ignition and emission control are correctly optimised and set.

The Exhaust Stroke ฀฀At the end of the power stroke, as much energy as possible has been extracted from the fuel and converted to mechanical work at the crankshaft. The next part of the working cycle is to reject the exhaust gases and their remaining heat energy from the cylinder. This process is controlled via the exhaust valves, which open around the end of the power stroke to vent the cylinder combustion gases to atmosphere via the exhaust system. Typically, the exhaust valves opens before the end of the power stroke to allow the remaining pressure energy in the end gases to assist in evacuating the cylinder.

Exhaust Valve The exhaust valve is opened via the engine valve gear and is synchronised with the engine operating cycle. Opening the exhaust valve before the end of the power

Timing diagram stroke assists in the process of evacuating

the cylinder volume efficiently. During the exhaust stroke, the piston rises from BDC to TDC, thus the cylinder volume decreases and this ejects the exhaust gases out through the exhaust valve. Generally, the inlet valve opens before the end of the exhaust stroke and this creates a certain amount of time when both inlet and exhaust valves are open. This allows the kinetic energy of the exiting exhaust gases to assist in drawing the fresh gas charge into the cylinder.

 Describe the effects of incorrectly set ignition timing.

 Look back over the previous section and write out a list of the key bullet points here:

ENGINE TERMINOLOGY AND SYSTEMS

Technical Terms The following are some of the technical terms that are used to describe features of the engine:

Engine capacity – This is the total, combined, displaced volume of all engine cylinders as a single value stated in units of cubic capacity. Generally this is given in cubic centimetres (cm3) or litres. In America, engine capacity is normally stated in cubic

inches (in 3 ) Swept volume – This is the volume of a cylinder bore between

the TDC and BDC piston positions, excluding the volume above the piston at TDC

Clearance volume – This is the volume above the piston at TDC, note that it is the volume of the combustion chamber itself.

Bore – This is the diameter of the engine cylinder. Stroke – This is the total linear distance travelled by the

piston in the bore between TDC and BDC positions. Note that it is twice the crankshaft throw.

Compression ratio – This is the total volume of the cylinder at BDC (swept + clearance volume), expressed as a ratio of the volume of the cylinder at TDC (clearance volume).

The information on cylinder dimension can generally be found in workshop or manufacturer manuals. In addition, these values can be measured directly or derived via calculations.

Swept Volume This can be calculated via the formula: (lπd 2 )/4,

d = cylinder bore and l = stroke. Note that units of bore and stroke must be consistent. Engine volume is mostly stated in litres by manufacturers but remember that 1000cc (cubic centimetres) equals 1 litre. The total engine displacement is the sum of all cylinders individual displacements.

Compression Ratio The formula used to calculate the compression ratio (CR) is:

CR = (Vs + Vc)/Vc Where CR = compression ratio

Vs = swept volume Vc = clearance volume Note correct order of preference when

carrying out this calculation (brackets first – BODMAS).

Torque Two common terms used when expressing engine performance characteristics are ‘torque’ and ‘power’. Torque is an expression

Torque and power relating to work and is a measure of the turning

force provided by the engine. Torque output can vary independent of engine speed and is a measure of the load on the engine. The units of torque are Newton metres (Nm) for SI units and Pounds/foot (lbs/ft) in Imperial units. Power is a derived unit and relates to the rate of work done, or the work done per unit of time. For an engine, the power is a product or torque and speed. Power output is given in Kilowatts (KW) or Horsepower (HP). Engine power is normally stated as measured at the flywheel, via

a dynamometer or brake, hence the term ‘brake horse power’.