Team-BHP - Do diesel cars require less frequent gear changes?

As the diesel engines produce high torque at low RPM, theoretically they should require less frequent gear changes (compared to similar size petrol engines).

But is it really the case?:confused:

Since I never drove a diesel car I wonder whether they are more comfortable for long drive.

The amount of gear change required mainly depends on gear ratios.
Since different cars have different gear ratios it won't be right to claim outright that diesel cars require less gear change.
However since diesel engines produce max torque at relatively low RPM compared to petrol cars so on two cars (one diesel the other one petrol) with same gear ratios yes diesel car will require less gear change.

As for second part of question

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Since I never drove a diesel car I wonder whether they are more comfortable for long drive.

Again comfortable will be dependent on the seating position, leg space available and other factors.
For driving yourself on highway in diesel car is fun for sure. Since torque is produced at low RPM you can shift to over drive early and continue to drive so for longer time.
Also diesel cars can tolerate poor quality fuel available on highways, though some newer common rail engines are equally vulnerable to adulterated fuel as petrol cars.

All things being equal, the sheer torque = driveablity of a diesel leads to fewer gear changes. Try driving an Octavia or a Mahindra SWB around town and you will know. The Mahindra 2.1 Peugeot is almost like an automatic in third gear within the city.

Then how come the Diesel cars pick up the speed a lot slower than petrol ones? Is the power output for diesel engine low at low rpm?

If that's the case, while you 'can' drive the car in a same gear for a wide range of speeds, it won't be a fun to do so. The poor pick-up would compel you to down-shift. That's why the Sumo is so much hated among the drivers.

I think, for driving pleasure, it has to be petrol. While, diesel is economical for long trips.

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Then how come the Diesel cars pick up the speed a lot slower than petrol ones? Is the power output for diesel engine low at low rpm?

Umm, drive a common-rail diesel. It will change a lot of your diesel perceptions.

Some one told me , since the diesel engine torque curve is often flatter than petrol engines', speed is less likely to fall down as quickly as in petrol engine, once you lift your foot off the accelerator. Does it mean more "riding" the brake on diesel cars in town compared to petrol cars?

In fact, I've observed many diesel Amby cab drivers, that they mostly drive on 2nd/3rd gear with just using brake/accelerator in city traffic (similar to what GTO said about driving like automatic in 3rd gear :)). Diesel engine doesn't stall even at very low RPM.

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Originally Posted by amu1983 (Post 389589)

Then how come the Diesel cars pick up the speed a lot slower than petrol ones? Is the power output for diesel engine low at low rpm?

I think, for driving pleasure, it has to be petrol. While, diesel is economical for long trips.

My dear friend, just drive a CRDI and you will know the difference, it is way way ahead of most petrol cars in pick-up to extent it's mind boggling. I drive around in city in 3rd most of the time and sometimes in 4th gear. I hardly need to downshift to 2nd.

Your second statement holds true. Meaning huge pick up without comprising on being economical;)

Diesel cars usually have taller gears up the gear shift thats why you need to down shift a lot lesser than its petrol siblings and GTOs right, try a CRD engine but dont start with a TATA crd....try the Hyundai or the Merc or even the BMW.

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Originally Posted by sbasak (Post 389659)

In fact, I've observed many diesel Amby cab drivers, that they mostly drive on 2nd/3rd gear with just using brake/accelerator in city traffic

Thats probably due to none of the other gears working in the first place; or familial knowledge handed down from their fathers/grandfathers abt the best way to drive an Amby taxi

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Originally Posted by sbasak (Post 389376)

I think (theoretically!), just like the amount of torque, another equally important factor that would decide how much each gear can cover is rpm RANGE in which it is produced. While higher torque in diesel cars calls for lower shifts as compared to petrol ones (comparable models, of course), the usable rpm range usually acts against it. In a way, the fact reflects itself in lower power number of diesel in spite of having higher torque. So it kind of gets balanced, but the net result may still favor one of them. I don't know since I too have hardly driven any diesel car. Also, I was talking about traditional diesel engines, things may be different for modern CRDIs as many have pointed out.

It is in general true that the diesels require lesser gear shift. Ok lets take the most basic of them, the Indica NA. At 45kmph you can be on 5th gear and puttering around happily. Engine won't complain. Try the same on a petrol, for that matter even the Xeta. Anything less than 55kmph will mean the engine lugging and if you can not steadily accelerate from then on the speed will soon reach 50 and you will have to shift back to 4th.

Just as GTO said, even the Indica is like an automatic on the 3rd gear... and if you are driving in city (speed varying between 15kmph and 45kmph) it actually serves the purpose of an automatic on the 3rd gear.

P.S.: Ok, on the 3rd gear at 15kmph if you have more than 2 people then you will have a problem else it pulls fine.