Understanding Torque and KW in diesels

[Ricof4e]

Gents,

Let's say I drive a Gobabis Toyota. Apart from it being the fastest diesel 4x4 known to man, what does the torque and kw figures actually mean?

For instance, if the torque figures are a gazillion NMs at 1500RPM, and the KW a gazillion more at 3500RPm - what does that mean?

Do I have the most power at 3500RPM or at 1500RPM?

Two scenario's come to mind:

1) If I am towing a V6 Hilux, do I try and keep the needle at 3500RPM?

2) If I find myself adjacent to any Alonso look-alike, how do I regulate gear shifts vis-a-vis the revs?

[XJ Junkie]

Alex explains it best. But essentially kw is a measurement of how fast an engine makes power & torque is the amount of force generated.

One of the best analogies I’ve heard is when a vehicle is compared to a Boxer. The kw is how fast you punch & torque is how hard you punch.

Turbo diesels are typically better at towing because diesel burns slower than petrol.

[Mad Manny]

Power is how hard you hit the wall.
Torque is how far you carry the wall after hitting it.

F1 cars need power more than torque.
A train needs torque more than power.

Ideally the power & torque, well most of it, should be available over a wide band & with a good overlap.

[Mad Manny]

Generally a Turbo Diesels Torque reading is 3x its power reading.
So a TD with 98kW will have close to 300Nm.

A NA Petrol usually has a Torque reading double the kW.
So a 100kW petrol will have around 200Nm.

Also the Power & Torque peaksare always A LOT lower in a Diesel than a Petrol.

Diesels struggle to rev past 4000rpm & very fewcan get to 5000rpm.
Many petrols hit peak power way above 4000rpm.

That's why I prefer Diesel, the power & torque is there in the band that you generally drive in.

[XJ Junkie]

What I don’t like about diesels is that the peak torque window is very narrow making it tricky. This is where big petrol engines excel because they may make less torque but it’s in a wider, more useable, window.

[ThysleRoux]

What I don’t like about diesels is that the peak torque window is very narrow making it tricky. This is where big petrol engines excel because they may make less torque but it’s in a wider, more useable, window.

Agreed!

Manny explains it fairly well - except for the fact that diesels' power "band" (rev range where it makes ANY power) is usually very narrow (only a few hundred RPM), which means that to keep the rev's within that range requires lots of gearshifts in changing terrain. A proper sized petrol engine will produce torque from low rev's - the 4.1 Chev motor of yore is a great example - depending on the version (Std or High Ouput) it produced peak torque at 1500 or 1800 rpm. The torque/power ratio in bigger petrol motors also generally exceed Manny's 2:1 figure. Rooitjiep produces 175kW and 400Nm (peaking at 3200rpm), for instance. The 4.0 I6 engine of the XJ and TJ made a decent 132kW and 304Nm (at 2500 rpm)

Normally aspirated diesels are even worse than TD's, which is why diesels need turbo's to operate properly. Turbo charge a similar sized petrol engine and it will beat a diesel in all areas, other than consumption - you need to feed 'em horses to keep them performing.

[grips]

Never underestimate big v8 petrol engines when it comes to torque. I can idle along the same dune drive with the Yank v8 Ranger at a happy 700rpm. In the v8 Cruiser I need to stay above 1400rpm to keep it moving. Even on the big diesel v8 low down torque needs the turbo to boost.

Have to sort of learn to drive the TD V8 in sand. Quickly learned you does not just put foot when you need horses, Reaction time on the diesel v8 much slower. Towing well with close to 600nm at hand it is hard to beat the Yank Ranger. I am afraid the V8 petrol is only beaten at the pump.

Having lots of lugging torque is nice in a diesel, having that same torque and double the horse power in a petrol v8 is just so much nicer

[Mike Nel]

So what would be the ideal amount of Kw's and ideal amount of Nm's into one engine? It has to viable though.

For example:

My FJ had 200Kw's and it was enough for what I did with it in the Dunes.
It had 380Nm's which wasn't enough for towing comfortably

The Ranger has 147Kw's which is ok, but not really enough for the Dunes
But 470Nm's which makes for a far more comfortable towing experience than the FJ

So where is the happy medium?

[Ricof4e]

The happy middle is a relatively small twin turbo diesel engine.

Ideally 2.3lt with the full 450nm available at 1500rpm(ish)

Very little turbo lag.

Buy a navara

[XJ Junkie]

So what would be the ideal amount of Kw's and ideal amount of Nm's into one engine? It has to viable though.

So where is the happy medium?

For me it isn’t about how much power, but rather where in the rev range it delivers peak power & how wide the rev range is.

Some of the best offroad engines ever made were straight 6 petrol engines that were specifically designed for off-roading in any type of terrain.

On sand diesels can be a lot of fun on slow technical obstacles because of the low down traction that they deliver.

[Quack]

Power (kW) is a function of Torque (Nm) and RPM
Torque is a function of force and distance (remember diesel's longer stroke because it relies on compression for combustion)
Those that dabble with motors will know that a stroker will always be more torquier than a square motor, even if it's a big Yank V8

Therefor you need to look at your torque curve to realise how effective it will translate to your application.

Petrol engines generally rev higher than diesels because the components are lighter as they do not have to be as strong as diesels, as the compression ratio is much lower than that of a diesel.

There are some modern small diesels that run happily past 5000 rpm

And then just as you think you understand all off this we need to look at the torque multiplying characteristics of the gearbox and how efficient this can be done!!

Ultimately the diesel will always win, for every litre of diesel that you burn, you get up to 15% more inherent energy! That is just the nature of the two beasts! But if you measure fun factor, you need to understand octane numbers!

Conclusion: I threw a bunch of random statements at you! At the end of the day we need to understand a lot more than just anecdotal observations.

If you want a daily runner and you do decent milage, get a modern diesel with a motor plan, but if you want to have fun in the dunes, speak to Uncle Grips and let him build you a petrol toy with catalogued parts!

[ThysleRoux]

So what would be the ideal amount of Kw's and ideal amount of Nm's into one engine? It has to viable though.

For example:

My FJ had 200Kw's and it was enough for what I did with it in the Dunes.
It had 380Nm's which wasn't enough for towing comfortably

The Ranger has 147Kw's which is ok, but not really enough for the Dunes
But 470Nm's which makes for a far more comfortable towing experience than the FJ

So where is the happy medium?

The mass of the vehicle and the gearing at the ideal towing speed will play a huge role - it is of no use to have oodles torque in a rev range which translates to 160 km/h and only a fraction at 100km/h. That is why the ideal engine will have a flat torque curve, with little variation over as big a rev range as possible.

[grips]

Power (kW) is a function of Torque (Nm) and RPM
Torque is a function of force and distance (remember diesel's longer stroke because it relies on compression for combustion)
Those that dabble with motors will know that a stroker will always be more torquier than a square motor, even if it's a big Yank V8

Therefor you need to look at your torque curve to realise how effective it will translate to your application.

Petrol engines generally rev higher than diesels because the components are lighter as they do not have to be as strong as diesels, as the compression ratio is much lower than that of a diesel.

There are some modern small diesels that run happily past 5000 rpm

And then just as you think you understand all off this we need to look at the torque multiplying characteristics of the gearbox and how efficient this can be done!!

Ultimately the diesel will always win, for every litre of diesel that you burn, you get up to 15% more inherent energy! That is just the nature of the two beasts! But if you measure fun factor, you need to understand octane numbers!

Conclusion: I threw a bunch of random statements at you! At the end of the day we need to understand a lot more than just anecdotal observations.

If you want a daily runner and you do decent milage, get a modern diesel with a motor plan, but if you want to have fun in the dunes, speak to Uncle Grips and let him build you a petrol toy with catalogued parts!

Now this is a good post Think Derick explained it very nice. For towing applications it is hard to beat a turbo diesel. Also the new 8 and 10 speed auto boxes keeping engines in the perfect power and torque ranges.

But then with 6.0l and 7.0l petrol and diesel engines you do not need fancy boxes any gear goes