two revolutions of the crankshaft during the four-stroke cycle.
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In the most popular valve timing diagram two circles, one inside the other, areused to represent the 720° of crankshaft rotation through which the crankshaft moves for a complete cycle. Each stroke is represented by an arc of 180° with induction and compression on the outer circle and combustion and exhaust on the inner circle. The valve opening and closing positions are marked and the duration of crankshaft rotation is displayed by a thicker line.
From the valve timing diagrams it can be seen that the valve opening and closing positions do not occur within the 180° of crankshaft rotation for each stroke of the four-stroke cycle. For instance, towards the end of the exhaust stroke, the inlet valve begins to open and this is before the exhaust valve has closed. The exhaust valve fi nally closes as the piston moves down on the induction stroke. The inlet valve closes as the piston is rising on the compression stroke. The exhaust valve opens before the end of the combustion stroke. The opening and closing positions of the valves are specifi c to individual engines and are matched to other design and performance requirements.
The terms applied to the valves when opening before and closing after the start of a stroke and when both valves are open together are called ‘lead’, ‘lag’ and ‘overlap’, respectively. The overlap position is often referred to as ‘valves rocking’ and can be used as a rough guide as to when a piston is at TDC.
All internal combustion engines have an induction, compression, expansion and exhaust process. For a four-stroke engine, each of these processes requires half an engine revolution, so the complete engine cycle takes two complete engine revolutions. That is, there is a working and a non-working (gas-exchange) revolution of the engine within the cycle. However, a two-stroke engine combines two of the processes in each half turn of the engine; thus, all processes are complete in one engine revolution and the engine has a power stroke with every revolution ( Fig. 2.9 ). In order to operate, the two-stroke petrol engine uses the crankcase (piston underside) for induction of the fuel/air mixture and transfer into the cylinder via ports in the cylinder barrel.
On the upstroke, the piston moves upwards towards TDC, and fuel/air charge trapped in the cylinder space above the piston is compressed and, around TDC, ignited by a spark. This is the beginning of the power stroke. As the piston rises
1 3
2 4
Figure 2.9 Two-stroke operating cycle: 1, compression; 2, induction; 3, combustion (power);
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Automobile mechanical and electrical systemsduring the upstroke, the volume in the crankcase increases and atmospheric pressure forces the fresh fuel/air charge into the crankcase (under the piston). On the downstroke the piston moves towards BDC as the power stroke begins; the expanding gases force the piston down the bore, producing torque at the crankshaft via the connecting rod. At the same time, the crankcase volume decreases and the fuel/air mixture is compressed under the piston. As the piston approaches BDC, the transfer port connecting the cylinder volume to the crankcase volume is uncovered by the piston. On the opposing side of the cylinder, the exhaust port is also uncovered.
This allows the fresh charge in the crankcase volume to transfer and fi ll the cylinder volume, at the same time forcing the exhaust gases out of the cylinder via the exhaust port. The effi ciency of this scavenging process is very dependent on the port exposure timing and the gas dynamics. Often the piston crown has a defl ector to assist this process and to prevent losing fresh charge down the exhaust. Note that two-stroke gasoline engines are normally lubricated via the provision of an oil mist in the crankcase. This is provided by oil mixed in with the fuel/air (premixed or injected); hence the oil is burnt in the combustion process, which produces excessive hydrocarbon emissions.
Two-stroke engines are generally more powerful for a given displacement owing to the extra power stroke compared to a four-stroke engine, but the problem is that the expansion stroke is short and volumetric effi ciency (how easy it is to get the gases in and out of the engine) is poor, so they are less effi cient. In addition, exhaust emissions are higher than from a four-stroke engine.
Some large static diesel engines are often two-stroke types ( Fig. 2.10 ). Note that all four operating processes are executed in one engine revolution (induction, compression, expansion and exhaust). The diesel engine requires a charge of air that is compressed to raise its temperature above the self-ignition point of the fuel. This air charge is supplied by an air pump or pressure charging device (turbo or supercharger). The pressurized air from this device passes into the combustion chamber via ports in the cylinder wall. The exhaust gases leave the
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combustion chamber via cam-operated poppet valves. The incoming chargeforces the exhaust gases out via these valves (the cylinder scavenging process). During the downwards movement of the piston, the hot expanding gases are forcing the piston down the bore, producing torque at the crankshaft. This is the expansion process. As the piston approaches BDC, the exhaust valve opens and the remaining pressure in the exhaust gas starts the evacuation of the gases in the cylinder via the open valves. As the piston moves further down to BDC, inlet ports are exposed around the bottom part of the cylinder bore, which allow the pressurized, fresh air charge from the air pump (or turbocharger) to fi ll the cylinder, evacuating the remaining exhaust gas via the valves and completing the exhaust and induction cycles.
At BDC, the cylinder contains a fresh air charge and the piston then begins to move up the cylinder bore. The inlet ports are closed off by the piston movement and the air charge is trapped and compressed by to the deceasing volume in the cylinder. At a few degrees before TDC, the air temperature has risen owing to the compression process and fuel is injected directly into the combustion chamber, into the hot air charge, where it vaporizes, burns, and generates thermal and pressure energy. This energy is converted to torque at the crankshaft via the piston, connecting rod and crankshaft during the downstroke.
Another variation on engine operation is the Wankel (the name of the inventor) or rotary engine ( Fig. 2.11 ). This engine has been used in a limited number of passenger car applications. The engine uses a complex geometric rotor that moves within a specially shaped housing. The rotor is connected to the engine crankshaft and turns within the housing to create working chambers. These are exposed to inlet and exhaust ports to allow a fuel/air charge in, compress it and expand it (thus extracting work), then evacuate the waste gases and restart the cycle ( Fig. 2.12 ). The rotor has special tips to provide a gas-tight seal between the working chambers. The movement of the rotor in this engine follows a path know as an epitrochoid.
No matter what design of engine, it has to be positioned in the vehicle. There are various confi gurations that manufacturers have used in the confi guration of their vehicle powertrains. The engine can be front, mid or rear mounted and can be installed in-line (along the vehicle axis) or transverse (across the vehicle axis) ( Fig. 2.13 ).
Figure 2.11 Rotary engine. (Source: Mazda Media)
Defi nition
Epitrochoid
A roulette traced by a point attached to a circle rolling around the outside of a fi xed circle.
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Automobile mechanical and electrical systemsThe engine mounting system is important as it supports the weight of the engine in the vehicle. In addition, it counteracts the torque reaction under load conditions. The mounting system has to isolate the vehicle from the engine vibrations. The engine mounts consist of steel plates with a rubber sandwich between to provide the vibration isolation ( Fig. 2.14 ). The mountings have appropriate brackets and fi ttings to fi x to the engine and vehicle frame. For a front-engine, rear-drive powertrain layout, the engine mounts are often at the centre position of the engine side, approximately at the engine centre of gravity ( Fig. 2.15 ). The engine mounts bear compression and shear forces in supporting the engine weight and torque. The rear of the engine is bolted to the transmission, which in turn is supported at the rear end via a rubber mounting system. This three-point mounting is very common for this powertrain confi guration.
For a front-wheel drive, transverse powertrain layout ( Fig. 2.16 ), the mounting system has to cope with weight of the engine, plus the torque reaction of the
Figure 2.12 Rotary engine cycle: starting with the top image, induction, compression, power,
exhaust. (Source: Wikimedia)
1 2 3 4
Figure 2.13 Typical positions for the engine: 1, front transverse engine FWD; 2, rear transverse
engine RWD; 3, front longitudinal engine FWD; 4, front longitudinal engine RWD