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Please explain the importance of Engine Compression Ratio.

In Text books I could find the following importances

  • Engine Cold starting Characteristics
  • Torque Generated
  • Fuel Consumption
  • Noise
  • Pollution.

But I am not able to understand the relationship between these characteristics and the compression ratio. Could you please give me more example/explanation on how these are related

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What is Compression Ratio:

  • An engine's compression ratio is the ratio of the volume of gas in the cylinder when the piston is at the top of its stroke (top dead centre, or TDC) to the volume of gas when the piston's at the bottom of its stroke (bottom dead centre or BDC). In other words, it's the ratio of compressed to uncompressed gas, or how tightly the incoming fuel/air mix is squeezed into the combustion chamber before it's ignited. The more it's squeezed, the more efficiently it burns and the more power is made. Note:If you want to know how the figures arrive check out this link

    How to calculate compression Ratio

How does it effect fuel economy:

  • The higher the ratio, the more compressed the air in the cylinder is. When the air is compressed, you get a more powerful explosion from the air-fuel mixture, and more of the fuel gets used. Think about it this way: If you had to be near an explosion, you'd probably choose to be near one somewhere outside, because the force of the explosion would dissipate, and it wouldn't seem as powerful. In a small room, however, the force would be contained, making it feel much more powerful. It's the same thing with compression ratios. By keeping the explosion in a smaller space, more of its power can be harnessed. By increasing the compression ration from 8:1 to 9:1, for example, you can improve fuel economy by about 5 to 6 percent.

How does it affect Pollution or emmision:

  • High compression ratios provide better burning of fuel and this reduce the waste gase produced by the engine , anythign higher than 16:1 will be probelm and create something called a knock. Lower compression ratios allow you to burn bad quality fuel or lower octane fuel thus increasing waste gases.

How does it effect engine cold starting characteristics:

  • Engines with high compression ratios produce excessive heat compared to lower compressed vehicles thus doing a cold start should not be a problem for these type of vehicles , in most motorcycles(with high compression ratio) not touching the throttle while doing a cold start is very effective. As for a lower compressed engine the case is the same for any normal engine you need it to heat up for a better start.

How does it affect performance:

  • This is a no brainer , higher compression ratios will provide much more power than lower ratios that is why F1 cars have extremely high compression ratios. This also allows engine to be more efficient at higher rpms. Lower compression ratios give lower output but are more easy to build,maintain and gernerally have better life.

Why would I go for higher compression:

  • Much better performance.
  • Higher "relative" fuel economy .
  • higher usable power throughout the rpms.
  • Better emissions.

Why would i go for lower compression:

  • Much much cheaper to build,run and repair.
  • Lasts much longer than counterpart.
  • Can be air cooled.
  • Less Noise,vibration.
  • Better real world fuel economy.
  • Runs on crappy fuel.
  • Does not Generate as much heat.

The last point is very crucial for motorcycles since the engine is very close to the legs of the user , a naked bike with high compression ratio will either be impossible to drive in the city or will need Proper liquid cooling.

Hope this helps.

  • Hi, I didn't quite get how low compression ratio will have a better "real world" fuel economy? Also, what exactly do you mean by "relative" fuel economy in case of high compression ratio? – Stark07 Apr 25 '17 at 9:38
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    @Stark07 Higher Compression means it will take less fuel to ignite technically so should result in Better Fuel Economy(diesel engines) but since vehicles with high compression ratios are not built with fuel economy in mind they ultimately end up giving lesser real world FE. (because of the torque advantage higher CR gives)Vehicle with lower compression ratios though technically less efficient are built with Economy in mind so the ultimately end up giving better Real world FE – Shobin P Apr 26 '17 at 11:13
  • Thank you for replying on a 2 year old thread! This makes more sense. So it's more about the target of the engine rather than the effects of the compression ration itself. – Stark07 Apr 27 '17 at 7:11
  • @Stark07 Exactly! – Shobin P Apr 27 '17 at 9:42

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