Everlasting Diesel vs Petrol

grogi

Inspired by the heated discussion in the 5008 thread, I decided to crunch some numbers and objectively compare how a diesel would compare to the petrol.

Because I wanted to link back to the 5008, I took the two engines discussed there - 1.2 PureTech 130 and 1.6 BlueHDI 120. Based on some charts for power/torque found the net and official spec from Peugeot I modeled the two.

PureTech: 130 PS @ 5500 rpm, 230 Nm @ 1750 rpm
BlueHDI: 120 PS @ 3500 rpm, 300 Nm @ 1750 rpm



But that is just dry figures, without any context. We need some more data to directly compare how they would behave...

I needed gearbox ratios, but could not find direct info for gearboxes used. I picked the ratios for Aisin AWTF-80SC gearbox (predecesor of AWF6F25, which is sold by PSA as EAT6). https://en.wikipedia.org/wiki/AWTF-80_SC

I wanted to calculate the velocity that relates to particular rpm on each of the gears and see how much power is available at that speed at that gear. So I played with the main ratio for each of the engines trying to match what I could see at Youtube. I am pretty confident that the ratios are within 10% correct now.

Let's throw it together on one chart and here we go:



Green curves represent petrol engine, each curve for each gear. Similarly orange are for the diesel.

So, what does it tell us? Actually a lot...

Let's see the maximum acceleration... When shifted optimally, the petrol is almost always faster...



Or let's take the standard overtaking test - 80-120 on 5th gear.

Driving 80 km/h on 5th gear petrol has 2100 rpm, diesel 1700 rpm. It seems that, damn you maths and physics - because it contradicts my previous beliefs, the diesel is faster - has more power on 5th gear from around 75 km/h and damages the petrol from around 90 km/h.
(It is consistent with the manufacturers spec, which gives overtaking times of 11.4s and 10.2s respectively).

But if you're not afraid of mangling with the stick or driving auto, the PureTech will overtake the BlueHDI when optimal gear (that means 2nd!! for the petrol, 3rd for the diesel) is selected... I have highlighted the relevant curves... Even with less optimal reduction - petrol to the 3rd and diesel to the 4th (so the maneuver can be finished without further shifting), the petrol would be faster.

Similarly if you are doing 80 km/h on the 6th (like many people will do, to save fuel) and wanted to accelerate without shifting, the petrol will be faster. But one really wants to get out of the 6th - at 80 km/h shifting down from 6th to 5th gives 50% (for petrol) to 100% (diesel) more power.



It seems that with manual gearbox and normally shifting human, a similarly powered diesel is faster. But pair that petrol with automatic and it will be faster. I am not ashamed to say that I am really surprised by the result. I was certain the results will be much closer.

There were a lot of assumptions (the gear rations are a bit different in life for sure) that were made. I had to estimate the torque curve for the PureTech - it mimicks the one from 110 version. I will be happy to correct the calculations and charts should more data is available...

That analysis is valid for those two engines. Should the torque curve looked different (exp. flat torque at the 1750-2500 range for the petrol, like in the TSI engines or 1.0 Ecoboost), the results would be different! So the answer is not universal and between different motors of similar power there will be different results for sure.

EndaHonesty

68 lost souls

Fairly interesting comparison. Would be interested in seeing other cars done out like this. Some interesting comparisons would be GTD vs GTI vs GTE (as well as Manual vs DSG)

grogi

OSI wrote: »

Don't think it'd really be fair to do a GTD/GTI/GTE comparison. In the OPs comparison there's only 10bhp in the difference, there's 60bhp between a GTD and a GTI.

Find the dyno charts and max speed of each, I'll throw it together anyway.

heroics

Out of interest do you feel like comparing the 530i vs 530D or a 318i vs 318D?

Micky 32

Having owned both diesel and petrol bhp for bhp they are practically as fast as each other despite the diesel redline being lower, nothing in it. That is my experience anyway. I had a 300bhp vrs petrol turbo and i now have a 290bhp biturbo diesel and going by certain markers on the road i can get it to the same speeds as i did with the petrol turbo of similiar bhp when flat out.

Nuw

Thanks OP very interesting to look at. What would awesome on top of this would be to find out consumption at certain ratios for both, just for the fun of it. Pretty sure it'd be a pain to find out though...

DaveyDave

I was actually reading about this the other day. People always say that if you overtake in a diesel it just goes whereas a petrol you need to put the foot down hard. As you pointed out this is because you generally need to drop a gear in the petrol as the power isn't there in the low revs. I think this is less of a problem in modern turbo petrols though.

It's all about the power curve and it's quite simple really. The i30 diesel I drive only puts out 75bhp at 2,000rpm which is when it changes gears unless the foot is down it goes to 3,000rpm where it's 108bhp and because it's an automatic when you floor it there's a switch under the accelerator that will downshift past 90% throttle. It makes sense then that cruising at 1,250rpm is feck all power especially how steep the power curve is in the 1.6 CRDI.

Even if you don't want to get technical about it...just don't labour your engine

Leonard Hofstadter

DaveyDave wrote: »

As you pointed out this is because you generally need to drop a gear in the petrol as the power isn't there in the low revs.

QFT. That's why petrol is almost always faster than a like for like diesel, they always have that bit more bhp and therefore they are faster. You might need to work the gearlever but it will ultimately be faster and changing gear in a decent petrol is no sacrifice as they have sweeter shifting boxes than diesels, and that's before we get onto all the other advantages of petrol; they have much wider and more linear power bands and therefore are much more tolerant of being driven in the wrong gear, they are quieter when you just want to toddle along but sound much nicer when you want to have fun, they are lighter so they handle better, they are much kinder to our lungs, they rev harder and they have better throttle response, which makes them more fun.

In short, they are better in every possible way than a like for like diesel*.

* With couple of notable exceptions, such as several BMWs, for example the current 330d/530d has a 3.0 six cylinder diesel with 261 bhp, the 330i/530i has a piddly little 2.0 four cylinder with just 248 bhp - 3 bhp, 1.0 litres and 2 cylinders less than the 330i/530i of 13 years ago had by the way. Similarly with the 118i/218i/318i, these models have 0.5 litres, 14 bhp and 1 cylinder less than their diesel equivalent (and also have 7 bhp, 0.5 litres and 1 cylinder less than models with the 18i badge had 16 years ago, so so much for progress). Also the gap between petrol and diesel narrows a fair bit when you put an automatic into a diesel because they beautifully disguise the narrow power band and a few other things that I don't like about diesels.

grogi

captainspeed wrote: »

QFT. That's why petrol is almost always faster than a like for like diesel

I think the difference in perceived performance of diesel engine goes ultimately to noise. In a civilian car, people don't want to run an engine around 4000 rpms when cruising the motorway. Because of that the ratios in a petrol driver-train of 6th gear are much longer that they could be - only to reduce the noise and fuel consumption. That stretches the gears below.

Because of that it is the 4th gear really corresponds (performance wise and flexibility wise) to 5th in a diesel.

To illustrate the above, I took the estimated acceleration of the previously mentioned 5008 1.6 BlueHDI 120 on 5th and 4th gear, and 1.2 PureTech 130 on 4th and 3rd gear...



As expected - the turbocharged petrol in the 3rd gear covers the more-less same velocities (30-140 km/h) as the diesel in the 4th (40-150 km/h), but is a bit faster. Similarly petrol in the 4th performs better than diesel in the 5th in every aspect, even covering wider velocity range (40-160 km/h vs 50-160 km/h).

If it wasn't mainly for the noise, the gearbox ratios in production cars could be a bit different. I've played with the ratios again to equalize the ratios between petrol and diesel drive-trains, not giving any consideration to the noise levels at the motorway. Below is the simulated acceleration levels of petrol (with modified gearbox) and the diesel (with production gearbox). Petrol would never be slower in any standard overtaking test. But this noise (and increased fuel consumption).



To sum up again - in a regular car, if a driver is not afraid to shift the gears and push the engine a bit higher (or is driving an auto), a turbocharged petrol it is faster. If a driver is more relaxed and doesn't fancy shifting that much - turbocharged diesel behaves better when overtaking.

Henry Ford III

You've way too much time on your hands.

p.s. I'm surprised how the torque curve falls away at relatively low revs in both petrol and diesel engines.

grogi

Last one, I promise

This is how a naturally aspirated petrol with 120 PS compares to the turbocharged diesel with same power... It simply is crushed without hope...

Comparing overtaking on the 5th gear makes no sense, even reduction from 5th to 4th does not help at all - diesel is much faster on 5th than N/A petrol on the 4th...

ct5amr2ig1nfhp

grogi, dragging up this thread. Solid work and fair play to you for your effort! do you still have the data readily available or have you binned it?

I am wondering how the 5008 1.5 bluehdi 130 holds up against both the petrol 1.2 and the older 1.6 bluehdi 120

grogi

Hudson Howling Storeroom wrote: »

grogi, dragging up this thread. Solid work and fair play to you for your effort! do you still have the data readily available or have you binned it?

I am wondering how the 5008 1.5 bluehdi 130 holds up against both the petrol 1.2 and the older 1.6 bluehdi 120

1.5 has slightly more puch around 3500 rpm. I don't think you'll feel any difference in normal usage between 1.5l 130 and 1.6l 120 bhp honestly.