BTDC, IVC 28 ATDC EVO 18 BBDC, EVC 6 ATDC

IVO 5 BTDC, IVC 28 ATDC EVO 18 BBDC, EVC 6 ATDC

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

ENGINE OPERATING DETAILS

Atmospheric Pressure The air above the Earth’s surface is a fluid that exerts a pressure on all points around it. This is due to the weight of the air acting down upon the Earth’s surface and this, in turn, is because of the earth’s gravitational force pulling it down. This creates a pressure is known as atmospheric pressure and is approximately 1 bar or 15 pounds per square inch

Naturally Aspirated Engine A naturally aspirated engine relies on atmospheric pressure to charge the cylinder with gas (air or air/fuel mixture) ready for the combustion process. As the piston moves down the cylinder (from TDC to BDC). The volume increases and this causes the pressure in

Inlet of the cylinder to reduce, becoming lower than

fuel-air atmospheric pressure. This creates a pressure

mixture difference between the inside and outside of the

cylinder, and due to this, the atmospheric pressure (the higher pressure) forces gases into the cylinder (where there is lower pressure) until the pressure is balanced. Note that any restriction to the flow of gas will reduce the effectiveness of the cylinder charging process

Volumetric Efficiency This is a measure of the efficiency of the cylinder charging process during the induction stroke. Theoretically, the cylinder should

be completely filled with a mass of

gas but in practice this never happens due to flow losses and inefficiencies. Therefore, the volumetric efficiency is a measure of the actual amount of gas induced compared to the theoretical amount (which is the mass required to completely fill the cylinder volume) and is expressed as a percentage. It is calculated as: (the actual mass of air / the theoretical mass of air) x 100%

Breathing or Aspiration The more efficiently the engine cylinders can fill with gas, the more air, or fuel/air, is available for the combustion process and this improves overall engine efficiency.

The process of getting gases into and out of the engine is known as ‘asp iration’ or ‘engine breathing’.

Petrol and Diesel Combustion in the engine cylinder takes place because of a chemical reaction between the carbon and hydrogen in the fuel and the oxygen in the air. This reaction releases energy

from the fuel in the form of heat that generates pressure in the cylinder to force movement of the piston. In order to achieve efficient combustion, the quality of the fuel/air mixture is important. That is, how evenly mixed the fuel droplets are in the induced air. Movement of the air as it enters the cylinder is important for this process and the requirements are different for petrol and diesel engines. The required air movements for each engine type are created by careful design of the components that form the inlet tract and combustion chamber.

Inlet Valve The inlet valve opens and closes according to piston position and controls the incoming gas charge into the engine. It generally remains open for a small time period after the piston has reached BDC (i.e. beyond the end of the inlet stroke). This allows the energy of the moving gas column in the inlet tract to assist in the cylinder charging process which helps to

increase volumetric (and engine) efficiency

The Compression Stroke ฀฀ After the combustion chamber has been charged with gas (air or fuel/air) during the induction stroke, the cylinder inlet and exhaust valves are both closed and seal the combustion chamber. The piston begins to rise in the cylinder, thus reducing the volume of the cylinder space and hence increasing the pressure of the trapped gas charge in the cylinder prior to combustion. The opening and closing of the valves is executed in sequence via the engine valve gear, synchronized with the 4-stroke cycle and piston position.

Piston Rings It is important that the closed cylinder is sealed properly to maintain the appropriate pressures in the cylinder during the working cycle. Any losses in pressure would significantly

reduce the efficiency of the engine. In order to seal the piston and bore, piston rings are fitted into radial groves near the top of the piston and provide the gas tight seal between the moving piston and the cylinder bore. When the cylinder volume is reduced during the compression stroke, the trapped gas is compressed and the amount of compression is known as the compression ratio. Compressing the charge prior to combustion allows more oxygen or fuel/oxygen in the cylinder than would otherwise have been available without compression and this improves combustion efficiency. 249

Temperature Rise During Compression (Petrol) ฀ During compression of the fuel/air mixture in a petrol engine, heat energy and kinetic energy (due to gas movement) are imparted into the mixture due to the reducing volume and rising pressure, This creates a significant temperature increase and the magnitude of this increase depends upon the speed of the compression process and the amount of heat rejected to the surroundings (via the cylinder combustion space, walls, head etc.). The temperature rise elevates to a point just below the self-ignition temperature of the fuel/air charge, which will combust at or above the flash-point when ignited via an external source (i.e. the spark plug). Note that if the temperature of the mixture was too high, spontaneous, self- ignition could occur and this would be a limiting factor for the maximum compression ratio in a petrol engine.

Temperature Rise During Compression (Diesel) ฀฀ In a diesel engine, the compression process must create sufficient energy to cause the temperature of the compressed gas (air) to rise above the self-ignition temperate of the fuel that is injected into the cylinder at the end of the compression stroke.

Air Turbulence ฀฀ Inlet charge movement is particularly important in a diesel engine, in order to ensure that the fuel droplets have sufficient oxygen for complete combustion. The required air

Direct… Indirect… flows during the induction and

compression processes are created by the design of the inlet tract and combustion space. Generally, there are two designs of combustion chamber in common use and these are named, as such, due to the position in the chamber where the fuel is introduced. They are known as direct and indirect injection

Direct Injection A Direct injection combustion chamber has a ‘bowl’ formed in the piston crown. This is designed to promote a tumble movement of the incoming air mass; this helps to ensure

good distribution of the fuel in the cylinder and reduced soot emissions.

Indirect Injection The indirect type combustion chamber incorporates a pre-combustion chamber within the cylinder head. The compressed inlet charge is forced into this chamber at Pre-combustion chamber and

Swirl chamber high-velocity and pressure. This

creates a swirl movement that ensures complete mixing of fuel droplets with air for maximum combustion efficiency. During the combustion process, the burning gases are ejected from this chamber with high pressure and energy. This ensures sufficient turbulence in the main combustion chamber for efficient combustion.

Compression Ratios The compression ratio of a direct injection engine is typically between 16 and 21:1. This is sufficient to raise the induced charge temperature for self-ignition of the

Compressi With fuel under all engine operating

conditions without creating excessive on ratios different combustion noise (or diesel knock).

vary…

engine types

Indirect injection engines have higher compression ratios of 22 to 25:1. This is necessary to generate the extra heat energy required due to losses via the increased surface area of the cylinder head. Diesel knock is less apparent in indirectly-injected engines as the energy release is more controlled and less spontaneous.

The Combustion or Power Stroke ฀ After compression of the inlet charge, combustion of the fuel creates heat and pressure energy, which is imparted on the piston to generate mechanical work. In a petrol engine, this process is initiated by the high voltage arc at the spark plug electrodes in the cylinder.

Combustion or Power Stroke The diesel engine is designed to produce compression pressures that generate sufficient heat in the cylinder to ignite the fuel as it is injected into the combustion chamber. This is

known as compression ignition (CI). Petrol engines are generally known as spark-ignition (or SI) engines. During the combustion stroke, the engine power output or work is generated, hence the name ‘power’ stroke in the 4-stroke cycle of induction, compression, power and exhaust. Engine combustion is a fundamental process in the operation of the engine. This process must be efficiently executed and controlled via the engine sub-systems (fuel, air, ignition etc.) to ensure best efficiency and performance, with minimum harmful exhaust emissions.

Complete Combustion of Fuel and Oxygen Combustion in the cylinder of an engine is a chemical reaction process between carbon and hydrogen in the fuel and oxygen present in the induced air. The carbon and oxygen combine to form Carbon

Dioxide (CO 2 ), the hydrogen combines with oxygen to form water (H 2 O). Nitrogen passes through the engine as long as the combustion chamber temperatures remain below critical limits.

Incomplete Combustion of Fuel and Oxygen If the combustion process is not efficient, incomplete combustion will result and this produces Carbon Monoxide (CO). If combustion chamber temperatures are high, oxides of Nitrogen are

produced (NO X ). These are both harmful pollutants and their emissions from Motor Vehicles are closely regulated and controlled by environmental protection agencies and bodies around the world.

Ignition Timing The combustion process should occur in a rapid but controlled manner. The flame propagation and energy release in the cylinder should have a predictable, stable behaviour depending on the

engine operating conditions. The timing of the spark ignition is critical to achieve appropriate energy release for maximum efficiency in the energy conversion process that takes place in the combustion chamber. The burn duration of the fuel varies according to engine conditions; therefore, the spark must be adjusted to occur at the correct time, according to these conditions, to get the optimum torque from the engine. The optimum spark advance for a given engine condition is known as Minimum Spark Advance for Best Torque (MBT)

Petrol The quality of Petrol is measured by a parameter called the ‘octane’ rating and this gives an indication of the fuels resistance to engine ‘knock’ or uncontrolled, spontaneous combustion, which causes engine damage. The higher the octane rating, the slower and more controlled the fuel burns and hence, the greater the resistance to ‘knock’. The octane rating of the fuel determines the limit of ignition advance for

a given engine speed and load condition. Therefore, it is particularly important to always operate the engine on the correct fuel, to prevent damage to the engine due to ‘knocking’.

Mixture Strength A chemically correct air and fuel ratio mixture must exist in order to ensure that sufficient oxygen is present to completely

Mixture ratio combust all of the fuel.

This is known as mixture strength and is ratio of air mass to fuel mass. For most fuels, the correct ratio is approximately 14.7 air mass to 1 part fuel mass. If more air is present then the mixture strength is known as ‘weak’. If less than a

14.7 air/fuel ratio, then the mixture strength is known as ‘rich’. Weak and rich mixtures are less than optimum for the engine, although under certain conditions the mixture strength is adjusted by the engine control system according to demand. For example, for full power a slightly rich mixture is 258