Quote:
Originally Posted by JimD
If you google up what Wikipedia says about the otto cycle (the technical term for a normal 4 cycle engine) you will see a reference to improved efficiency with turbo charging. That is just one possible reference, it is an established fact from an engineering point of view that there is an efficiency advantage to turbocharging.
But what you guys are saying has merit too. If you use the extra power of the turbo motor, your mileage will be lower. The efficiency gain is small so the first principal of more power out requires more fuel in still applies. In the case of how it was implemented by BMW in the 135i, any efficiency gain from the turbo was offset by the gearing raising the rpm of the motor. The advantage went totally to performance, not fuel economy.
Weight is also a factor but the weight of the extra pieces for turbo charging is not going to be a big factor. Most of us do not track gas mileage based upon how many passengers we have. There is some small effect but we ignore it. The turbo weighs a lot less than another passenger.
I'm not sure about the point that the boosted engine isn't always on boost is a gas mileage factor. I thought of that too. But the 135i engine is supposed to build boost at just over 1,000 rpm. So there should be some boost nearly all the time. Maybe not at part throttle.
I am confident the key factors to reduced mileage in the 135 are the gearing and use of the added power. If the 128 and 135 were geared the same and driven the same, the mileage would be equal or the 135i would be better. Driving a 135i like a 128i might be pretty difficult, however.
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It will build boost at 1,000RPM if you want to accelerate, sure. It will also use a lot more fuel than an equivalently-throttled NA engine of the same displacement, because more air is going in (so you can add more fuel). But maintaining speed, you're not building boost because you don't need to or want to. At a constant speed and off boost, there is really no difference. A better example to illustrate your point would be the Mustang, which offers an NA 3.7l V6 and the Ecoboost 2.3l 4-cylinder, making comparable power (300 in the V6 and 310 in the I4). The Ecoboost gets 3mpg better than the V6 across the board while having more peak power and more useable power lower in the rev range. The efficiency gains in the real world come from using a smaller engine to make the same power as a bigger engine.
This entire thread is so theoretical that it's really not relevant to the real world at all. Not saying the discussion isn't interesting, I just don't think you can extrapolate any conclusions drawn here to explain the mileage you see in your car (128i or 135i). But since we're just bullshitting here, I am also pretty sure that the container ships streaming across the Atlantic full of high pressure fuel pumps and water pumps needed to keep the 135i fleet running are negating any theoretical fuel efficiency gains from said 135is!
