Friday, March 28, 2008

Mileage Boom!

I'm not referring to the increasing awareness about depleting fuel levels. I'm going to talk about the misconception with most drivers that driving in a lower than optimum gear will help fuel economy. Engines are tuned to be driven in a certain RPM range both for power and economy of fuel usage. Above or below that range, you're not going to get the best mileage out of your car. Most cars, in addition to being not so fuel-efficient in very low RPM ranges, produce an irritating 'boom' inside the cabin. This is not any fault. All engines produce vibrations in varying frequencies at some point in their RPM range. The engineers cannot dial out those vibrations. What they can attempt to do is limit those vibrations to the RPM range that is seldom used by the driver. In city conditions, we often lie in the region of 2500-4000 RPM. So, the engineer might look at confining the engine vibrations either below 2500 or above 4000, if it's a city car. In Innova, the range seems to be below about 2500 RPM.

I recently travelled from my college in my friend's Innova and it was nice. Except for one thing. That irritating booming noise present below the specified RPM. I'm not sure if a better driver could've worked the engine better--it was being driven by my friend's driver and you can't expect drivers to know about all the technical stuff. The same effect is also present in my uncle's Scorpio (non-CRDI unit).

In this respect, I applaud the Fiat 1.3 MultiJet unit in the Swift. It's butter smooth in this regard. The Innova 2.5 litre unit is much smoother higher up the RPM range, however.

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4 comments:

Sriram said...

What happens if we dont maintain the specified limit apart from mileage point of view?

S R I R A M said...

In most conditions, nothing worrying will happen. At worst, 'transmission snatch' might occur. It's a grunting noise from the gearbox when the car is going too slow for the specified gear.

Sriram said...

Will it affect the Performance of engine?

S R I R A M said...

I guess you're referring to transmission snatch. Less skilled drivers will actually struggle to keep the engine running when the snatch occurs. So, in a way, we can say that it does affect the performance. Hope I was clear.