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The Four-stroke-cycle spark-ignition (petrol) engine
- Induction Stroke: The inlet valve is open. The movement of the piston creates suction pressure that induces (sucks in) fresh charge of air and atomised petrol.
- Compression stroke: Both the inlet and exhaust valves are closed. The piston moves upward. The charge is progressively compressed to 1/8 to 1/10 of the original volume, increasing the charge pressure and temperature.
- Power stroke: Both the inlet and the exhaust valves are closed. Just before the piston reaches the TDC during the compression stroke, a spark plug ignites the charge. When the piston reaches the TDC, the charge begins to burn, rapidly raising the pressure and temperature and forcing the piston to move downward in the power stroke.
- Exhaust Stroke: At the end of the power stroke, the exhaust valve opens. Because the cylinder pressure is much higher than atmospheric (about 4 bars). The remaining burnt gases in the cylinder will be pushed by the movement of the piston upward in the exhaust stroke.
The Four-stroke-cycle compression-ignition (diesel) engine
The induction and exhaust strokes are the same.
3. Compression Stroke: Both the inlet and the exhaust valves are closed. The piston moves upward. The charge is progressively compressed to 1/12 to 1/24 of its original volume, raising the pressure to 30-50 bars.
4. Power Stroke: Just before the end of the compression stroke, diesel fuel is injected, vaporised by the heated charge. The mixture is ignited. The burning of the mixture raises the pressure inside the cylinder very rapidly and forces the piston to move away from the cylinder head.
Comparison between the spark-ignition and compression-ignition engines
Thermal efficiency: Petrol engines can have thermal efficiency ranging between 20% and 30%. Diesel engines have improved efficiencies, between 30% and 40%.
Noise: Diesel engines are noisier. The combustion process is quieter in the petrol engine and it runs smoother than the diesel engine.
Cost: Due to their heavy construction and injection equipment, diesel engines are more expensive than petrol engines.
The two-stroke-cycle petrol engine
- Induction and exhaust stroke: The piston moves down the cylinder and initially uncover the exhaust port (E) releasing the burnt gases to the atmosphere. Such a movement also compresses the charge in the crankcase. Further movement of the piston uncover the transfer port (T) allowing the compressed mixture to be transferred to the inside of the cylinder pushing out any remaining of the burnt gases.
- Compression and power stroke: The piston moves in the direction of the cylinder head, sealing off all ports and compressing the mixture. Further movement of the piston increases the volume in the crankcase, creating suction so when the inlet port is uncovered fresh charge is sucked in the crankcase. Just before reaching the TDC, a spark plug ignites the compressed mixture, raising the pressure and temperature of the mixture very rapidly. The burnt gases expand forcing the piston to move down the cylinder.
Comparison of the two and four-stroke cycle petrol engines
Theoretically, the two-stroke engine should develop twice the power of the four-stroke engine for the same cylinder size but actually the factor is 1.3 because the induction –exhaust stroke of the two-stroke engine is less effective. The cooling load is greater for the two-stroke engine and it is thermally less efficient than the four-stroke engine. The two-stroke engine has fewer working parts so it is cheaper to manufacture.
Two-stroke cycle diesel engine:
The piston moves away from the cylinder head and when it is half way down its power stroke the exhaust valves open allowing the burnt gases to escape through the exhaust valves. When the piston moves down near the end of the power stroke, the inlet ports are uncovered, allowing fresh air from the blower to be admitted. The piston moves upward toward the cylinder head sealing off the inlet ports and helping the fresh air to push any remaining burnt gases through the exhaust valves and the exhaust valves close. The piston continue to move upward compressing the charge of air and raising the temperature and the pressure to about 30 to 40 bars. Before the piston reaches the TDC fuel is injected into the charge. The heated charge vaporises the fuel and ignites it. The rapidly burning mixture raises the pressure and temperature very rapidly inside the cylinder and forces the piston to move away from the cylinder head in the power stroke.
Piston & Connecting-rod assemblies
Piston-Ring action:
Piston ring can be divided into two groups:
i)Compression rings, whose function is to seal the space between the piston and the cylinder wall so that gas can not escape.
ii)Oil-control rings, whose main purpose is to control the amount of lubricant passing up to the top of the cylinder walls.
Compression-ring action:
The piston ring is designed to expand radially outward when fitted in its groove so the ring will tend to spring outwards to apply pressure on the cylinder wall.
On the upward compression stroke, the compressed charge will move between the groove and the ring side faces, press the ring against the lower groove of the piston. This provide a very effective compression seal without leakage.
On the downward power stroke, piston acceleration is greater than that of the ring so that the upper groove and the ring side faces will be held firmly together to form the seal.
Oil-Control-ring action
During the crankshaft rotation, oil is splashed from the big end bearing on to the cylinder walls. The oil control scraper ring performs two functions: first, it regulates the amount of oil passing to the combustion chamber and secondly, it distributes a film of oil over the cylinder surface to lubricate the cylinder wall and the compression rings.
On the piston’s upward stroke, the lower face of the ring will be held firmly against the lower groove so that the upper face of the ring will scrape a proportion of the oil. The excess oil will accumulate in the clearance space of the groove until it overflows through the drillings to the sump.
On the piston downward movement the ring will snap over to the top of the ring groove. The sharp edge of the working face of the ring will scrape the oil down the cylinder wall. The surplus oil accumulates in the space of the groove and overflows through the drillings.
Piston and piston-ring working clearances
- Piston-ring side clearance:
Is the gap between the ring and land side faces. With insufficient clearance, the expansion of the lands will wedge the rings in their grooves and could destroy the oil film and cause overheating. A lose fit will cause the ring to flutter. This hammers the ring against the groove faces, producing rapid groove wear. Ring side clearance can be checked by removing the ring from the piston and rolling it around the outside of the piston in its groove, suitable size of feeler gauge can be slipped between the ring and the groove to check the clearance.
Typical minimum ring-side clearances for pistons between 6 and 12 cm in diameters are as follows:
Compression ring: Petrol 0.05 mm, Diesel 0.06 mm
Oil –control ring: Petrol 0.04 mm, Diesel 0.04 mm
Piston-ring joint clearance
A clearance must be allowed at the piston-ring joint to compensate for the expansion, which takes place from cold to hot working temperature. Insufficient clearance will cause the ring ends to buckle, expanding the ring against the cylinder wall, affecting the oil film and can cause overheating. Rings with large gaps may cause a loss of compression.
Typical minimum ring-joint clearance are as follows: Water-cooled four-stroke engines 0.03 mm per cm diameter. Air-cooled four-stroke engines 0.04 mm per cm diameter.
Piston-skirt-to-bore clearance
The correct clearance between the skirt and the cylinder wall is necessary to eliminate piston slap when the engine is cold. To check the clearance, insert the feeler blade into the cylinder bore for its full length, then slide the corresponding piston into the bore so that it traps the feeler blade at its largest diameter then hold the piston and pull the feeler.
Piston and connecting-rod Gudgeon pin Joints
The piston and connecting-rod are coupled together by a Gudgeon pin , which is supported in holes bored in the piston at right angles to the piston axis at about mid-height position, the central portion of the Gudgeon pin passes through the connecting-rod small-end eye.
To secure the connecting-rod and Gudgeon-pin in position, the connecting-rod small-end faces are polished with emery cloth and heated evenly with an Oxy-accetylene torch (230 – 320 ºC) then the Gudgeon-pin is forced through both the piston and the small-end eye until it is centrally positioned. The small-end then cools and shrinks tight over the pin.
Crankshaft Construction
Main Journals: are the parallel cylindrical portions, which are supported by the plain bearings.
Counterbalance Weights: are attached or integrated with the crankshaft. Their function is to counteract the centrifugal force created by each individual crankpin and its webs.
Crank-webs: The cranked arms of the shaft, which provide the throws of the crankshaft are known as crank-webs. Their purpose is to support the big-end crankpin.
The flywheel: The flywheel serves three main purposes:
i)to support the clutch assembly and to transmit the drive between the crankshaft and the gearbox by means of friction between the friction face of the flywheel and the clutch driven-plate.
ii)To provide a carrier wheel for the ring gear when the engine to be started.
iii)To store energy on the power stroke so that it will be given out on the three idle strokes, this being necessary to reduce crankshaft speed fluctuation throughout each cycle of operation.
Valve Timing Diagrams and Camshaft Drives:
The inlet valve opens before TDC for the following reasons:
- To prevent excessive cam-follower shock loads and spring vibration, the inlet valve is opened very gradually.
- Due to its inertia, it takes some time for the charge to enter the cylinder after the inlet valve opens.
- To make use of the partial depression in the combustion chamber caused by the outgoing gases.
The inlet valve closes after BDC
To maximize the air charge entering the cylinder and make use of the inertia of the incoming fresh charge particularly at medium to high engine speed. At low speed, the inertia is insufficient to oppose the upward moving piston so that a portion of the newly arrived charge will actually be pushed back and return to the induction manifold.
Exhaust valve opens before BDC for the following reasons:
1)Toward the end of the power stroke the burning process slows down because the burnt gases suffocate and prevent the mixing and burning of the unburnt charge so that the loss of power is small.
2)To take advantage of the kinetic and pressure energy of the exhaust gases to clear the cylinder before the end of the power stroke.
Exhaust valve closes after TDC
To take advantage of the exhaust gases. The momentum of the outgoing exhaust gases leave a vacuum that induces fresh charge to enter the cylinder and simultaneously push any remaining exhaust gas out.
Crankshaft to camshaft drive
With the four-stroke cycle engine, the cycle of events of the inlet and exhaust valve opening and closing is performed by the camshaft in one revolution, but the piston strokes- induction, compression, power, and exhaust are completed in two crankshaft revolutions. Consequently; for the camshaft timing cycle to be in phase with the crankshaft angular movement, the camshaft has to turn at half crankshaft speed, that is a 2:1 speed ratio. The crankshaft to camshaft drive may be transmitted by three methods: chain, belt, or gear.
Chain drives are more expensive than belts. They are noisy, but they last longer. Timing gear trains are particularly suitable for medium to large diesel engine applications where reliability is essential. In all cases the pulley-wheel of the camshaft has twice the teeth of the crankshaft pulley-wheel.
Setting belt tension
Adjustment of the belt can be carried out by loosening the jockey idler locknut and pushing it against the smooth face of the belt with a moderate thumb pressure at the midpoint between the crankshaft and camshaft pulleys.
Poppet-valve operating mechanism
A poppet valve resembles a cylindrical stem with an enlarged mushroom disk at one end. The stem of the valve is situated in a guide hole. When the valve stem is moved in and out, the valve disk head will open and close.
Camshaft with push-rod and rockers
That valve-mechanism is made up from the following components:
a)a camshaft
b)a cam follower
c)a push rod
d)a rocker arm
e)a rocker shaft
f)a return spring
g)a poppet-valve
Cooling Systems
Engine heat distribution and the necessity for a cooling system
The energy released from the combustion of fuel in the cylinder is dissipated in roughly three ways:
35-45% heat energy doing useful work on the piston.
30-40% heat expelled with the exhaust gases
22-28% heat carried away by heat transference
The importance of the cooling systems
If the cooling was not effective, the heat-flow rate through the metal will be low and the temperature of the inner surfaces will rise to a point where the heat destroys the lubricating properties of the oil film on the cylinder walls. Simultaneously, thermal stresses will be established, which may distort the cylinders.
Methods of Heat Transfer
1)By conduction through solids or stagnant fluids
where A is the area of heat transfer, K is the thermal conductivity of the material, δ is the thickness of the material, and T1-T2 is the temperature difference.
2)By convection through the movement of fluids
where A is the area of heat transfer, h is the heat transfer coefficient, and T1-T2 is the temperature difference.
3)By radiation (no medium is required)
where A1 is the surface area of the body emitting the radiation, T1 is the temperature of the body emitting the radiation, T2 is the temperature of the body receiving the radiation, ε1 is the emissivity of the body emitting the radiation, F1-2 is the view factor from body 1 to body 2, and σ is Steven Boltezmann constant = 5,67 10-8 W/m2 K4
Types of cooling systems
i)Direct air-cooling, where cool circulating air is made to come in contact with the exposed and enlarged external surfaces of the cylinder and head and thereby dissipate their heat to the surrounding air.
ii)Indirect cooling (liquid cooling), where a liquid coolant is used to transmit the heat from the cylinder and head to the radiator. Movement of air through the radiator then extracts and dissipates the unwanted heat to the surroundings.
Direct air-cooling system
If direct air-cooling is to be used, the surface area of the outside walls of both the cylinder and the head must somehow be enlarged to anything from five to fifteen times the plain cylindrical surface area. Fins are used to increase the external surface area of the cylinder. The length of these fins will be greatest where the cylinder is hottest-near the cylinder head and will progressively reduce toward the cooler-operating crankcase.
Description of an air-cooled system
Air-cooled engines mounted on a motorcycle frame are usually exposed to the surrounding atmosphere. They rely on the natural air stream caused by the forward movement to circulate air around the cylinders, head, and crankcase.
For multi-cylinder engines, controlled air-cooling is usually achieved by incorporating a fan, which blows fresh air over the external finned surfaces of the engine. To improve the effectiveness of the blown air, the sides of the finned cylinders and heads are enclosed by a sheet metal. The shape of the sheet metal guides the forced convection current around all the cylinders and provide a direct exit after the air has extracted and absorbed the heat from the engine.
Heat transfer in an indirect liquid-cooled engine system
The heat released from the burning of the atomised mixture of air and fuel is transferred in all directions to the metal walls of the combustion chambers, cylinders, and pistons by direct radiation, by convection currents and then by conduction through a stagnant boundary layer of gas and a film of oil to the metal walls.
Due the difference in temperature between the inner and outer cylinder walls, heat will be conducted through the metals. It is then further conducted through a thin stagnant boundary layer of liquid to the coolant liquid in the passages around the cylinders.