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Many of the engine measurements and performance characteristics a technician should be familiar with. What follows are some of the important facts of each. Bore and Stroke: The bore of a cylinder is simply its diameter measured in inches (in.) or millimeters (mm). The stroke is the length of the piston travel between TDC and BDC. Between them, bore and stroke determine the displacement of the cylinders. When the bore and stroke are of equal size, the engine is called a square engine. Engines that have a larger bore than stroke are called oversquare and engines with a larger stroke than bore are referred to as being undersquare. Oversquare engines offer the opportunity to fit larger valves in the combustion chamber and use longer connecting rods, which means oversquare engines are capable of running at higher engine speeds. But because of the size of the bore, the engines tend to be physically larger than undersquare engines. Undersquare engines have short connecting rods that aid in the production of more power at lower engine speeds. A square engine is a compromise between the two designs. The crank throw is the distance from the crank shaft's main bearing centerline to the connecting rod journal centerline. The stroke of any engine is twice the crank throw ( +++17). Displacement A cylinder's displacement is the volume of the cylinder when the piston is at BDC. An engine's displacement is the sum of the displacements of each of the engine's cylinders ( +++18). Typically, an engine with a larger displacement produces more torque than a smaller displacement engine; however, many other factors influence an engine's power output. Engine displacement can be changed by changing the size of the bore and/or stroke of an engine. Calculation of an engine's displacement is given in Section 8. +++17 The stroke of an engine is equal to twice the crank throw. Crank throw Crank C L Rod journal C L +++18 Displacement is the volume the cylinder holds between TDC and BDC. TDC BDC Stroke Bore --- PERFORMANCE TIPs: The throw of a crankshaft determines the stroke. The length of the connecting rod only determines where the piston will be as it travels through the stroke. Therefore, it’s possible that the piston may reach out above its bore if a crankshaft with a longer stroke is installed with standard connecting rods. The correct combination of pistons with a higher piston pin hole must be used to prevent damage to the engine. Often the bore of an engine is cut larger to incorporate larger pistons and to increase the engine's displacement. Doing this increases the power output of the engine. However, this will also increase the engine's compression ratio. The compression ratio may also be increased by removing metal from the mating surface of the cylinder head and/or the engine block or by installing a thinner head gasket. Care must be taken not to raise the compression too high. High compression ratios require high-octane fuels and if the required fuel is not available, any performance gains can be lost. Use this formula to determine the exact compression ratio of an engine after modifications have been made: CR _ total cylinder volume with the piston at BDC _ the total cylinder volume with the piston at TDC The volume at BDC is equal to the cylinder's volume when the piston is at BDC plus the volume of the combustion chamber plus the volume of the head gasket. The volume of the head gasket is calculated by multiplying its thickness by the square of the bore and 0.7854. The volume at TDC is equal to the volume in the cylinder when the piston is at TDC plus the volume of the combustion chamber plus the volume of the head gasket. ------------- +++19 A gasoline engine is only about 25% thermal efficient. Exhaust loss 1/3 of input Input, gasoline 100% Output = approximately 1/4 of input Radiator loss 1/3 of input Radiant loss = 1/10 ----------- Compression Ratio: An engine's stated compression ratio is a comparison of a cylinder's volume when the piston is at BDC to the cylinder's volume when the piston is at TDC. The compression ratio is a statement of how the air-fuel mixture is compressed during the compression stroke. It’s important to keep in mind that this ratio can change through wear and carbon and dirt buildup in the cylinders. For example, if a great amount of car bon collects on the top of the piston and around the combustion chamber, the volume of the cylinder changes. This buildup of carbon will cause the compression ratio to increase because the volume at TDC will be smaller. The higher the compression ratio, the more power an engine theoretically can produce. Also, as the compression ratio increases, the heat produced by the compression stroke also increases. Gasoline with a low-octane rating burns fast and may explode rather than burn when introduced to a high compression ratio, which can cause pre-ignition. The higher a gasoline's octane rating, the less likely it’s to explode. As the compression ratio increases, the octane rating of the gasoline should also be increased to prevent abnormal combustion. Engine Efficiency: One of the dominating trends in automotive design is increasing an engine's efficiency. Efficiency is simply a measure of the relationship between the amount of energy put into an engine and the amount of energy available from the engine. Other factors, or efficiencies, affect the overall efficiency of an engine. Volumetric Efficiency --Volumetric efficiency de scribes the engine's ability to have its cylinders filled with air-fuel mixture. If the engine's cylinders are able to be filled with air-fuel mixture during its intake stroke, the engine has a volumetric efficiency of 100%. Typically, engines have a volumetric efficiency of 80% to 100% if they are not equipped with a turbo- or supercharger. Basically, an engine becomes more efficient as its volumetric efficiency is increased. Thermal Efficiency --Thermal efficiency is a mea sure of how much of the heat formed during the combustion process is available as power from the engine. Typically only one-fourth of the heat is used to power the vehicle. The rest is lost to the surrounding air and engine parts and to the engine's coolant ( +++19). Obviously, when less combustion heat is lost, the engine is more efficient. Mechanical Efficiency --Mechanical efficiency is a measure of how much power is available once it leaves the engine compared to the amount of power that was exerted on the pistons during the power stroke. Power losses occur because of the friction generated by the moving parts. Minimizing friction increases mechanical efficiency. +++20 The relationship between horsepower and torque. TORQUE (LB-FT.) 1,000 1,500 2,000 2,500 3,000 3,500 4,000 500 +++21 (A) Typical valve timing for an Atkinson cycle engine. (B) Typical valve timing for a conventional four-stroke cycle engine. Notice that the intake valve in the Atkinson cycle engine opens and closes later. Exhaust valve open--Exhaust valve open--Intake valve open--Intake valve open Torque versus Horsepower: Torque is a twisting or turning force. Horsepower is the rate at which torque is produced. An engine produces different amounts of torque based on the rotational speed of the crankshaft and other factors. A mathematical representation, or graph, of the relationship between the horsepower and torque of an engine is shown. This graph shows that torque begins to decrease when the engine's speed reaches about 1,700 rpm. Brake horsepower increases steadily until about 3,500 rpm. Then it drops. The third line on the graph indicates the horsepower needed to overcome the friction or resistance created by the internal parts of the engine rubbing against each other. Brake horsepower is a term used to express the amount of horsepower measured on a dynamometer. This measurement represents the amount of horsepower an engine provides when it’s held at a specific speed at full throttle. Horsepower is also expressed as SAE gross horsepower, which is the maximum amount of power an engine produces at a specified speed with some of its accessories disconnected or removed. SAE net horsepower represents the power produced by an engine at a specified speed when all of its accessories are operating. ------------ Advancing intake valve timing closes the intake valve sooner. This effectively increases engine displacement and produces more power. Atkinson Cycle timing keeps the intake valve open well into the compression stroke. This effectively reduces engine displacement, which minimizes fuel consumption. TDC VVT-i o 2° 18° 15° 105° 72° 34 +++22 Toyota's VVT-i (variable valve timing with intelligence) changes the engine from a conventional four-stroke cycle to an Atkinson cycle according to the vehicle's operating conditions. ------------- Atkinson Cycle Engines: An Atkinson cycle engine is a four-stroke cycle engine in which the intake valve is held open longer than normal during the compression stroke. As the piston is moving up, the mixture is being compressed and some of it pushed back into the intake manifold. As a result, the amount of mixture in the cylinder and the engine's effective displacement and compression ratio are reduced. Typically there is a "surge tank" in the intake manifold to hold the mixture that is pushed out of the cylinder during the Atkinson compression stroke. Often the Atkinson cycle is referred to as a five-stroke cycle because there are two distinct cycles during the compression stroke. The first is while the intake valve is open and the second is when the intake valve is closed. This two-stage compression stroke creates the "fifth" cycle. In a conventional engine, much engine power is lost due to the energy required to compress the mixture during the compression stroke. The Atkinson cycle reduces this power loss and this leads to greater engine efficiency. The Atkinson cycle also effectively changes the length of time the mixture is being com pressed. Most Atkinson cycle engines have a long piston stroke. Keeping the intake valve open during compression effectively shortens the stroke. How ever, because the valves are closed during the power stroke, that stroke is long. The longer power stroke allows the combustion gases to expand more and reduces the amount of heat that is lost during the exhaust stroke. As a result, the engine runs more efficiently than a conventional engine. Although these engines provide improved fuel economy and lower emissions, they also produce less power. The lower power results from the lower operating displacement and compression ratio. Power also is lower because these engines take in less air than a conventional engine. Hybrid Engines --Many hybrid vehicles have Atkinson cycle engines. The low-power output from the engine is supplemented with the power from the electric motors. This combination offers good fuel economy, low emissions, and normal acceleration. Some Toyota Atkinson cycle engines use variable valve timing to allow the engine to run with low displacement (Atkinson cycle) or normal displacement. The opening and closing of the intake valves is con trolled by the engine control system ( +++22). While the valve is open during the compression stroke, the effective displacement of the engine is reduced. When the displacement is low, fuel consumption is minimized, as are exhaust emissions. The engine runs with normal displacement when the intake valves close earlier. This action provides for more power output. The control unit adjusts valve timing according to engine speed, intake air volume, throttle position, and water temperature. Because this system responds to operating conditions, the displacement of the engine changes accordingly. In response to these inputs, the control unit sends commands to the camshaft timing oil control valve. A controller at the end of the camshaft is driven by the crankshaft. The control unit regulates the oil pressure sent to the controller. A change in oil pres sure changes the position of the camshaft and the timing of the valves. The camshaft timing oil control valve is duty cycled by the control unit to advance or retard intake valve timing. The controller rotates the intake camshaft in response to the oil pressure. An advance in timing results when oil pressure is applied to the timing advance chamber. When the oil control valve is moved and the oil pressure is applied to the timing retard side vane chamber ( +++23), the timing is retarded. Miller Cycle Engines-- An Atkinson cycle engine with forced induction (supercharging) is called a Miller cycle engine. The decrease of intake air and resulting low power is compensated by the supercharger. The supercharger forces air into the cylinder during the compression stroke. Keep in mind that the actual compression stroke in an Atkinson cycle engine does not begin until the intake valve closes. The super charger in a Miller cycle engine forces more air past the valve and, therefore, there is more air in the cylinder when the intake closes. The Miller cycle is efficient only if the supercharger uses less energy to compress the mixture than the piston would normally need to compress it during a normal compression stroke. This is an obstacle for engineers because to drive a supercharger requires approximately 10% to 20% of the engine's output. The latest Miller cycle engines control the action of the supercharger so that it’s only used when it’s better for compression and is shut down when piston compression is best. --- +++23 Oil flow for the VVT-i as it advances and retards the valve timing. Vane -attached to intake camshaft ECM--ECM Oil pressure ADVANCED TIMING RETARDED TIMING Oil pressure Rotating direction Vane -attached to intake camshaft Rotating direction -- +++24 A European four-cylinder passenger car diesel engine. --- Intake Compression Power Exhaust A four-stroke diesel engine cycle. Next: Diesel Engines Prev.: Engine Classifications Home Article Index top of page |
