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Unformatted text preview: Sarah Thomas November 23, 2010 ME476 Broch 1. How faster and slower burning cycles affect operation in an SI engine Variations in an engine cause faster or slower burning cycles with affect engine operation. In an SI engine spark timing is relative to TDC, this affects the pressure development which then affects the power of an engine and the imep. For ideal timing, the fuel/air mixture should ignite just before TDC so the engines combustion cycle is centered around TDC (burning angle is typically around 40 or 60, depending on engine speed). If combustion process is ill-timed, negative effects will occur. If the start of combustion is too late the peak pressure is reduced which also reduces expansion work, this directly effects engine performance. If the start of combustion is too early gases push back on the piston as it is coming up in the compression stroke, generating negative work and thus reduces torque. The point in time that provides the maximum brake torque is called MBT Timing. If torque was plotted against timing, (starting out below MBT), when the spark timing is advanced torque will increase until MBT timing is reached, after which any advance will cause torque to decrease. The top of the curve is somewhat flat, so the percentage increase in torque drops off as spark advance approaches MBT. With a lower burning velocity overall burn angle increases therefore spark timing needs to be advanced. When at an idle the residual gas fraction is higher with a long burn time, thus a long overall burn angle requires a more advanced spark. Maintaining an efficient spark timing is important because cycle variations can limit engine performance, e.g. fast burning cycles are prone to knock and slower burning cycles are more likely to have a higher residual gas fraction. 2. Briefly describe a CFR engine. Support your explanation with an outside reference/citation The Cooperative Fuel Research (CFR) engine is used for testing, research, and instruction in the performance of fuels in an internal combustion engine. This test engine is a 4-stroke over-head valve with a compression ratio between 3 and 30. It can vary due to a mechanism that can raise and lower the cylinder and cylinder head relative to the...
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This note was uploaded on 01/25/2011 for the course ME 476 taught by Professor Broch during the Spring '10 term at Nevada.
- Spring '10