Off-Road Chapter 6

Off-Road Chapter 6 - Chapter 6 Engine Design General...

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1 Chapter 6 Engine Design General Considerations ± The first step in designing a new engine is to choose the desired rated power output and rated speed. The load factor must be considered in choosing the speed. ± Load Factor = Average Power Output Maximum Power Output General Considerations con’t ± Engines in tractors, heavy-duty trucks, and other working vehicles have a much higher load factor than automotive engines. ± An automotive engine typically uses only a small fraction of its maximum available power while cruising at typical speeds on a highway, but may need to accelerate the engine to very high speed and use maximum power for brief periods.
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2 General Considerations ± For example, an automobile engine might produce its maximum power for a brief period while accelerating to 5000 rev/min when the car is passing another vehicle on a two-lane road, but might have a normal load factor of 0.3 while cruising at 2000 rev/min. Conversely, a tractor engine might run at 2000 rev/min with a load factor close to one for long periods of time while pulling an implement in a field. General Considerations con’t ± After the rated speed and power output are chosen, calculate the engine displacement that would be required to produce the power output at an acceptable p bme level. Experience with other engines has shown the p bme levels of 700 to 900 kPa are reasonable. General Considerations con’t ± After the engine displacement is chosen, the designer must decide on the number of engine cylinders needed to achieve the required displacement with reasonable piston size. Then the displacement per cylinder can be calculated. ± Choice of the bore-stroke ratio of an engine involves a design compromise. Smaller bore- stroke ratios permit higher compression ratios and the consequent higher combustion efficiencies.
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3 Engine Timing, Firing Orders, and Intervals ± An engine will not run unless the following events are timed to the rotation of the crankshaft: 1. Opening and closing of the valves 2. Firing of the spark plugs in SI engines 3. Start of fuel injection in diesel engines ± Also, in a multi-cylinder engine, the cylinders must fire in the proper order Engine Timing, Firing Orders, and Intervals con’t ± Cylinders are numbered from the belt- pulley (front) end of the engine toward the flywheel (rear) of the engine. ± In-line type engines have the cylinders numbered in the order in which their connecting rods are attached to the crankshaft. Engine Timing, Firing Orders, and Intervals con’t ± V-engines have two numbering conventions. Cylinders can still be numbered in the order
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Off-Road Chapter 6 - Chapter 6 Engine Design General...

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