Statement: A turbo diesel is NOT more powerful than a turbo petrol. Neither is a turbo petrol any less powerful than a naturally aspirated (NA) engine.
A very common misconception about turbo diesel is that torque gives you that crazy low rpm and this makes it more powerful than its petrol counterpart.
A second misconception about turbo diesel (and often the turbo petrol) is that at highway speed, you run out of steam, especially at 150+km/hr.
These are both NOT true.
Yes, the large amount of torque at low rpm is partly the reason why turbo diesels drive so well in town. However, this has nothing to do with having enough or not enough torque.
1 hp is 1 hp whether you are having 1 hp 1Nm of torque or 1hp 0.1Nm of torque, only difference is engine speed.
This means, a turbo diesel will have the same acceleration as a petrol engine of the same power.
But why does the turbo diesel feels so much faster at lower speed. Well, it is because power is a function of torque and rpm (torque x rpm). Quoting wikipedia...
Power is the work per unit time, given by
This means, the turbo diesel, which has good torque at low rpm, is able to produce more power when compared to a petrol engine at the same rpm. This makes it great for city users, especially many drivers who think that reving their engine beyond 3000rpm will cause the engine to explode.
However, take the petrol engine to higher revs and both will have matching power and matching performance. Using the mock up chart in Fig. 1, both the turbo diesel engine and turbo petrol engine in the chart will be rated as 200 hp by the manufacturer. However, if we look at what happens at a sane city driving engine speed of 2000 rpm, we see that the turbo diesel will produce 90 hp while the turbo petrol only produces 75 hp.
In order for the turbo petrol engine to match the output of the diesel engine at 2000rpm, we will need to rev the petrol engine nearer to 2500 rpm, a good 500 rpm more.
 |
| Fig. 1 Mock up power to rpm chart of diesel vs petrol turbo. |
BUT WAIT! Didn't we just mention that turbo diesel has more torque? If a petrol engine must be taken to higher rpm to have same power output as diesel engine, then wouldn't that mean that if we take the diesel engine to the same higher rpm as petrol engine, we get EVEN MORE power?
Yes, this is right in theory.
However, reality is a harsh world.
Firstly, torque of the diesel engine tapers down earlier.
Secondly, the petrol counterpart can get more rev. Therefore, say if comparing a 200hp turbo diesel pulling at 4500rpm and a 200hp turbo petrol pulling at 4500rpm, chances are, their power output are similar, but the petrol probably has another 1500rpm more before red line.
Going back to the example chart in Fig. 1, we can see that a turbo diesel has 200hp output at 4000 rpm. The same 200 hp rated petrol engine only has 175hp at 4000 rpm. Yet, at 4500 rpm, both have the same power output. While the diesel engine will red line at 4500 rpm, the petrol still has another 1500 usable rpm (red line 6000 rpm).
WTF? But we said power is a function of torque and rpm, how can the petrol engine in the example above have 200hp at 4500rpm and still only have 200hp when you rev it higher.
Isn't this playing cheat by comparing a more power petrol engine to a less powerful diesel? Surely that petrol engine will have more power than 200hp at say 5000 or 5500 rpm?
Nope. The reason is that with turbo petrol, manufacturers are able to control your boost pressure. This means, the ecu just drop boost (and torque) after 4500rpm and your power will stay a constant 200hp from 4500rpm to 6000rpm.
Isn't it amazing? Actually, this is just a trick to make you buy the more premium engine (e.g. BMW 520i vs 528i, BMW 520d vs 525d, Mercedes e200 vs e250), when in reality, the only difference is just how the boost is controlled.
Then what about the second misconception? Turbo diesel and turbo petrol run out of steam at highway speed.
NOPE. This again, is simply because you choose to buy the cheapo version of the engine where power output is highly restricted (see wall of text above).
Seriously, using a bit of common sense, we can understand that 300hp from a diesel, petrol, or even steam engine, is 300hp.
What is happening is that the good low end power of the turbo diesel (and some petrol) makes it feel like a much bigger engine than it really is. E.g. a 2L turbo diesel will drive like a 3.5L NA in the city. Very seldom will you need to rev it beyond 2500rpm.
However, because that tiny turbo diesel (or turbo petrol) is really, still just a tiny engine, the power output peaks early and then flat out.
On the other hand, the much bigger NA engine has good linear power that continues to increase as you push the rev higher.
This means, while the 2L turbo diesel will have about the same power as a much bigger NA engine at lower rev, it lose out at higher engine speed.
As an example, we have the mock up power chart of 4 engines in fig. 2. A 200 hp turbo diesel, a 200 hp turbo petrol, a 200 hp NA petrol, and a larger 300 hp NA petrol engine.
Comparing the 200hp turbo petrol and the 300 hp NA petrol engine, we can see power outputs are similar around town. However, once on the freeway where you are trying to play, the turbo petrol "runs out of steam" because the power out reaches a plateau of 200hp after 4500 rpm. The 300 hp NA however, continues to increase it's power output as the rev builds up beyond 4500 rpm. This means, the larger NA engine will just PULL away.
 |
| Fig. 2 |
On the other hand, if we drives the 200hp turbo petrol and 200hp NA petrol back to back, we may find the 200hp rating on the NA petrol to be over rated. This however, is NOT true. The manufacturer is not telling a lie when rating the NA at 200 hp. You just forgot to read the @xxxx rpm behind the hp rating.
A real life example can be seen below in the dyno chart from http://minkara.carview.co.jp/en/userid/224462/blog/32378862/
 |
| Fig. 3 |
The solution to all these problem? Buy a turbo, but buy the large displacement one.
*Disclaimer: We are really over simplifying the problem here. In reality, there is always the gearing issue to consider. Moreover, while the dyno says we have a certain amount of power at certain rpm, in reality, we DO NOT get this when driving. This is because that power is only available IF AND ONLY IF you apply full throttle at that engine speed.
