Why do diesels accelerate so slowly even with high torque?

[Bicolor]

Hmm my diesel car (2500cc NA, not turbocharged) must have rather weird gearing. Pick-up is only average, although I can get ahead of 70% of HK drivers if I'm the first at a traffic light. The fifth gear is immensely flexible though, and the acceleration from 90km/h to 150km/h (did it once only, on a very long straight highway) effortless, with just the slightest pressure on the accelerator. Didn't try anything above that speed and never will, so cannot tell how much more it can go. On the other hand, flooring the accelerator makes it pick up slower than a light gentle acceleration - this is probably the "running out of revs" others mentioned?

 

[Mini-itx]

Hmm my diesel car (2500cc NA, not turbocharged) must have rather weird gearing. Pick-up is only average, although I can get ahead of 70% of HK drivers if I'm the first at a traffic light. The fifth gear is immensely flexible though, and the acceleration from 90km/h to 150km/h (did it once only, on a very long straight highway) effortless, with just the slightest pressure on the accelerator. Didn't try anything above that speed and never will, so cannot tell how much more it can go. On the other hand, flooring the accelerator makes it pick up slower than a light gentle acceleration - this is probably the "running out of revs" others mentioned?

 

flooring accelerator does makes car accelerator slower compared to gentle and gradual stepping. It happens to both petrol and diesel cars, esp. NA ones. This is due to a drop in air velocity entering the engine and engine running too rich.

[Bicolor]

flooring accelerator does makes car accelerator slower compared to gentle and gradual stepping. It happens to both petrol and diesel cars, esp. NA ones. This is due to a drop in air velocity entering the engine and engine running too rich.

 

Not exactly, in virtually all petrol vehicles I've driven, many of which were small 1300cc NA cars, I often use a hard flooring of the accelerator for overtaking, sometimes dropping a couple of gears if a real quick burst of power is needed. In my diesel car, though, hard pressure doesn't have much effect, with or without a gear change.

[Mini-itx]

Not exactly, in virtually all petrol vehicles I've driven, many of which were small 1300cc NA cars, I often use a hard flooring of the accelerator for overtaking, sometimes dropping a couple of gears if a real quick burst of power is needed. In my diesel car, though, hard pressure doesn't have much effect, with or without a gear change.

 

What I mention applies only if you are at rest idling at traffic light. You try just flooring the accelerator while you are at rest at see the effect. Btw, when I say gradual stepping, I don't mean go slowly.

 

It actually bogs the car down rather than make it accelerate faster. Btw, it no longer affects new cars because the throttle is electronically controlled. Only cable ones.

[Ferraricalifornia]

diesel engines were designed for fuel economy as well as for heavy commercial vehicles, to move heavy loads and many people (buses) easily....they were not meant for performance....

 

it's only until recently, thanks to Euro emission standards, that engineers find it apt to fit a turbo-charger to diesel engines, to meet emission standards.

 

suddenly, horsepower and torque figures surge way pass the petrol alternatives.

 

Modern turbo-charged diesels can outrun an equivalent engine cc NA car (both in terms of acceleration and top speed)....even though the prominent of quick upshifts for diesel engine is still there, thanks to a much lower red-line and tighter gear ratios.

 

i guess the only reason why you'd still go for a petrol powered car is the engine noise it makes (much livelier and you can rev the engine till 6,000-8,000rpm for all you care as compared to 4,000-5,000rpm in a diesel)

 

and as well as a petrol engine is much more refined and quieter at idle....

[Happily1986]

In order to compare the petrol and diesel engine, one should look at the fundamentals. The diesel engine is designed with torque in mind with long bore stroke. Since diesel detonates not by spark but on compression, the long stroke is ideal since it allows the momentum of the piston to be maximised i.e. there is a cumulative effect with regards to the volume expansion. Mechanical work is thus maximised;

 

W = P * change in volume

 

Due to the lengthened bore stroke, the bore frequency thus naturally will have to be retarded. It is this retardation that limits the power output i.e. the power that spins the wheels at an increasing rate.

 

Most conventional petrol enginee (Ottis for e.g.) are designed with power in mind. Bore stroke is shorter in comparison and thus bore frequency can be much higher. It is much easier to extract BHP from petrol engines.

 

I think someone mentioned about century sprint timings with respect to torque and BHP. As I mentioned in my earlier posting, torque and BHP works hand in hand. Torque comes in first to provide the initial shove i.e. thrust which is akin to momentum then BHP comes in to generate pickup.

 

In theory, it is hard to obtain a century sprint of < 8s because of drag. Weight lightening aside, Drag as a dissipative force has a positive co-relation with the velocity of the car. At low speeds, this relationship is linear but at subsequent higher speeds, this relationship becomes a power function. Drag increases much much faster than the increase in velocity. Thus to keep up the acceleration, a increasing portion of power has to be devoted to overcoming drag.

 

The aerodynamic profile of the car becomes very important such as frontal area and drag coefficient. A car with a very efficient aerodynamic profile requires lesser power to keep it trundling along at a fixed speed. This voracious power requirement is why the Veyron has 4 turbochargers. Other than being there in order to force more air into the combustion chamber, they are there because you need a damn lot of power to produce the kind of acceleration required to break the 7 or 8s ceiling.

 

[Ferraricalifornia]

but too much torque also not good....

 

wheel spins would be noticeable on rainy days (where there's less traction between the tires and roads),

 

and the pin-back to your seat force makes car rides in city traffic all the more tiring and irritating....

[Mustank]

no wonder my pigkarp so slow

[Happily1986]

The automakers usually map the ECU so that not all of the Torque is released at slight throttling. Makes drivign a tad easier too. Beats having to feather the throttle all the time.

[Mustank]

i think my pigkarp got no ecu

[Ferraricalifornia]

The automakers usually map the ECU so that not all of the Torque is released at slight throttling. Makes drivign a tad easier too. Beats having to feather the throttle all the time.

 

 

most definitely....

 

and most cars now have that delay of .5-1s whenever you touch the throttle to move off....

 

it's not that the car isn't responsive....

 

but rather, it's a safety response, so that the driver don't kuku, ram into the car infront of it....

 

too much torque is not good....

[Daniu85]

would u choose to have a car with high torque and average bhp or a car with average torque and high bhp

[Joseph22]

would u choose to have a car with high torque and average bhp or a car with average torque and high bhp

 

look at Golf TSI 1.4 average BHP and high torque

the point is, what are the torque cruve and BHP curve...

[Mini-itx]

would u choose to have a car with high torque and average bhp or a car with average torque and high bhp

 

I would choose high torque and average BHP.

 

I don't travel at anywhere near 200 thus having high BHP is quite useless to me.

 

But I do need torque frequently as I need to accelerate fast to overtake cars or cut from lanes to lanes w/o affecting traffic or come out from side roads asap.

[Cerano]

I would choose high torque and average BHP.

 

I don't travel at anywhere near 200 thus having high BHP is quite useless to me.

 

But I do need torque frequently as I need to accelerate fast to overtake cars or cut from lanes to lanes w/o affecting traffic or come out from side roads asap.

 

at highway speeds your diesel will be out of grunt!!!

 

 

[Cerano]

look at Golf TSI 1.4 average BHP and high torque

the point is, what are the torque cruve and BHP curve...

 

1.4TSI torque and bhp definitely not averagev

[Xtrocious]

I don't call this "slow"...

 

The new Jaguar XF 3.0 Turbo Diesel

 

The specs as follows:

 

Engine: 2993cc V6 24v turbodiesel, 271bhp @ 4000rpm, 443lb ft @ 2000rpm

Transmission: Six-speed automatic, rear-wheel drive

Performance: 5.9sec 0-62mph, 155mph, 42.0mpg, 179g/km CO2

How heavy / made of?: 1820kg/steel

 

It's able to pull the 1820kg car from 0-62mph (0-100km/h) in 5.9s

 

 

[Cerano]

I don't call this "slow"...

 

The new Jaguar XF 3.0 Turbo Diesel

 

The specs as follows:

 

Engine: 2993cc V6 24v turbodiesel, 271bhp @ 4000rpm, 443lb ft @ 2000rpm

Transmission: Six-speed automatic, rear-wheel drive

Performance: 5.9sec 0-62mph, 155mph, 42.0mpg, 179g/km CO2

How heavy / made of?: 1820kg/steel

 

It's able to pull the 1820kg car from 0-62mph (0-100km/h) in 5.9s

 

look at the hp... its higher than 3.0L NA petrols