F0ul Thoughts

Mild mannered on the outside, Free thinker on the inside

Why a bigger engine might be more fuel efficient

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This post is an idea I had a while back, and I wanted to make it public just to see if I get any useful feedback.

In the world of motors, there are a few big numbers that get bragged about. The most popular, often misunderstood term is brake horsepower, BHP. You also have Cylinder Capacity, CC, Top Speed in Miles Per Hour MPH, and once in a blue moon, someone will mention the famous Torque in pounds per foot lbs/ft.

Motorcycles are a good place to see this in action. Taking the example of the difference between a 600cc motorcycle and a 750 one. a 600 will be 100-108 bhp, 750 115-125 bhp and that is normally all the difference you will hear. So what happens when you find a 600 with 130 bhp, what does that mean?

The only way to understand any of this is to explain how each number interacts with each other.

An engine generates power by making a fixed amount of fuel explode. That is what the CC is about. A bigger engine takes a larger amount of fuel in each cycle of power generation.

The amount of power created by the process is based on how often this process happens in time. This is measured in Revolutions Per Minute, or RPM. At 10,000 RPM, a bike might be creating 120 BHP. However, if you can increase this to 20000 RPM, you could get 250 BHP like a racing bike. This is however, very expensive because to make all the parts of the engine be able to move that quickly requires extremely light weight but strong components.

The higher the BHP, the more power the bike has to be able to move quickly, although the top speed is regulated by the weight of the bike and the gearing of the gearbox, and the aerodynamic shape of the bike.

Finally, there is the magical torque figures. This figure is about effort in a rotational force. Imagine you needed to turn a bolt, and had a 1 foot long spanner. If you placed a 1 pound weight on this spanner, and it was enough to turn the bolt, that would be 1 lb/ ft of torque.

In practical terms, torque is what starts a vehicle moving. It is also not related to how fast the engine is generating power, but more about its CC and how efficiently it is converting that fuel to power. Therefore, a 750cc bike will always have more torque than a 600cc.

Why does this matter? In real world usage, people are not racing around at high rpm everywhere for another reason. The more times you convert your fuel to power in a minute, the more fuel, measured in miles per gallon, MPG, you will use. Therefore, if you have a small engine, with a high bhp figure at a high rpm, but great fuel economy at a low rpm, you are going to get frustrated when you notice the fuel gauge and your speedometer moving in opposite directions at the same speed!

In the real world, the most important figures are the ones that relate to how you use your vehicle. If you like doing 100 mph everywhere, but want to save fuel, you want a vehicle with great fuel economy at that speed.

Suddenly, physics shows that it really isn’t possible to have everything!


Written by f0ul

December 27, 2012 at 11:15 am

Posted in Motorbikes

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