Quote:
Originally Posted by maanma
Many small engines go out of optimal range in highway speeds.
Worst ever experience is diesel 1.5 dci 18 mpg of Renault kangoo running on 3400 rpm at 80 mph.
Peugeot 107 (with city optimized Toyota KR1-FE 1.0 gasoline) does 31-33 mpg at 75mph. That is 134-125 mpg / seat. Compared to Renault Grand Espace 3 which will go 181 mpg / seat at 80 mph.
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Odd. The Kangoo is rated at 43-44mpg USA. (Of course, IIRC, european fuel mileage estimates are overly exagerated)
Wouldn't it be more of a transmission gearing problem than an engine problem? Consider:
- Unless climbing a steep grade or driving insanely fast, city driving can and will use more power than highway driving.That's because it takes a lot more power to accelerate a vehicle at every stop than to cruise on an open highway. A minivan could cruise at speeds of 75mph and only need 30hp/25kW or less.
- Gasoline engines are most efficient near full throttle, and diesels usually are most efficient at full throttle. Of course that efficiency is RPM dependent, usually getting that best efficiency at low RPM's.
- However, because of this, nearly all engines are too powerful to be able to be kept at their best efficiency while cruising along a highway even if you have high enough gearing. You need an overpowered engine in order to accelerate, because accelerating from stops in a passenger vehicle with just a little over 30hp wouldn't be a great experience.
Note this graph below found in one of Engineering Explained's videos. It uses BSFC (Brake Specific Fuel Consumption) to show how much fuel a typical gasoline engine uses.
- Note that the best fuel consumption or efficiency is the red circle marked 200. This circle is in the higher load area getting near full throttle and is also lower RPM's.
- Note that the vehicle is most efficient in 6th gear since that puts more load on the engine while lowering the RPM's at the same time.
- Also remember that in a diesel engine that red, highest efficiency circle would likely be placed higher in a higher torque/full throttle area.
- If the engine were more powerful, those spots representing the load/RPM's for each gear would be lower. This would place them farther from optimal efficiency.
- Now let's compare a small engine, but low geared setup. Imagine if 3rd gear on this graph were your top gear. Even though a smaller engine would raise it up to a higher load, it's still wound up at too high of RPM's in order to get good fuel mileage.
Conclusion: It's not the engine size that's the problem, it's the gearing isn't letting the engine cruise at low enough RPM's. You'd get better fuel milege with taller gears that make you smash that accelerator to the floor just to cuise at highway speeds than you would a bigger engine that just so conveniently comes with higher gearing.
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