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Engine braking tests in real life - diesel vs. petrol Jimny

07 Jul 2018 06:51 - 07 Jul 2018 07:32 #194078 by Bosanek

Since I (still) have two Jimnys in possession (one petrol and one diesel), I was very curious to compare the performance of two engines.
Both engines have the same power (63 kW), but that's the only similarity between them. Everything else is a large difference.

Quick engine spec. comparison:
  • Diesel engine has almost double the maximum torque at almost 2.5 times lower engine RPMs.
  • [Diesel engine has about twice as much compression ratio.
  • [Diesel engine is intercooled and turbocharged, while petrol engine has ordinary atmospheric intake.
  • [The gearboxes and differentials in two vehicles have completely different gearing ratios.
  • [Etc


Because of all this, I expected that the diesel Jimny will have significantly better performance when descending downhill on engine braking (without any foot or hand brake application)

"Better performance" in this context means "accelerating slower downhill and achieving lower maximum speed for the same stretch of downhill road".

Also, it is a common general conception that diesel vehicles perform much better on engine braking than equivalent petrol vehicles.

Test paremeters:

Requirements for making a valid comparison:
  1. Both vehicles should be empty of any cargo.
  2. Both vehicles should have the same driver.
  3. Both engines should be warmed up to operating temperature.
  4. Both vehicles have to use the same gearbox gear (for example: 2nd low).
  5. Both vehicles should have the same tyre size.
  6. Both vehicles should have the same tyre pressures.
  7. Both vehicles should have the same starting point, the same travel path and the same ending point.
  8. The starting point, the entire path and the ending point should all be on a significant downhill slope.

Note: The ending point is not the point in which the vehicle is stopped. The stopping of the vehicle has to commence further down the road. The ending point of the test section is a predetermined point on the road at which the final measurements are taken (time it took the vehicle to reach it from the starting point, vehicle speed and engine RPM).

My test setup fulfilled all those requirements except that the petrol vehicle had 3% larger tyre diameter (tye profile 205 / 75 / 15 compared to stock 205 / 70 / 15 on the diesel), putting the petrol vehicle at a slight disadvantage.

Test setup:

I performed the tests on two locations, and each test was conducted three times with each vehicle.

Both locations were paved streets with significant and varying downhill gradients. The length of the travel path was about 100-150 meters.
I used 1st high 2WD gear on the first location. I also tried using 1st high 4WD gear, but I got essentially the same results as in 2WD mode. Using 1st low gear there proved to be impractical. It just was not steep enough for that gear.

The second location has a super extreme gradient (people have major difficulty walking down that street when it's dry, and nobody except a few daring goats use the street when it's wet). Cars can climb up that street only if given a good running start.

Test results:
  1. In all of the tests which I performed, the petrol vehicle took approximately the same time to descend the test path as the diesel vehicle (actually it took the petrol vehicle one second less in the low range tests!).
  2. Also, the speed of the petrol vehicle at the ending point was approximately the same as the speed of the diesel vehicle.
  3. Another observation is that the acceleration curve of both vehicles was roughly the same (both of them increased their speed at a similar rate as they moved along).

For example, in the 1st location, where I used 1st high 2WD gear, both vehicles took about 10 seconds to descend and their final speed was about 30 km/h.
In the second location, where I used 1st low 4WD gear, both vehicles took about 27 second to descend and their final speed was about ... well 10 km/h (it's hard to measure such a low speed).


I find these results quite surprising, as they show that both vehicles have the same effective engine braking performance!

I can only guess that their performance downhill is the same because Suzuki intentionally "equalized" them by choosing corresponding gearing ratios in the gearboxes and the differentials. But I am not that technically competent to immediately jump onto that conclusion.
In other words, I am not certain if gearing ratios in a petrol vehicle can compensate for significant engine performance deficit against the diesel.

I would certainly like to read others' opinions about these conclusions and whether my test setup and conduction was a proper one in the first place, and also any comments about the conclusions.

One could even argue that the engine braking performance of the petrol vehicle is even slightly better than the performance of the diesel vehicle, because the petrol vehicle managed to match the diesel vehicle even though it had larger tyres at its disadvantage (larger tyres cause a vehicle to roll faster downhill).

The petrol engine did rev to much higher RPMs during the descends than the diesel engine (for example to 5500 compared to 3500), but that is incomparable anyway because of different engine construction.

I have recorded videos of most of the descents, which show the display of the speed and RPM gauges in the instrument cluster during the conduction of the tests. But I don't have the time now to process and upload the videos.
Last edit: 07 Jul 2018 07:32 by Bosanek. Reason: Additional comments
The following user(s) said Thank You: Riccy, ow3x3xpr3ss, Gadget

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08 Jul 2018 08:10 - 08 Jul 2018 08:22 #194114 by Bosanek
I forgot to state engine specifications:

Petrol Jimny:
Made in 2012
Engine Suzuki M13A VVT, 1328 cm3
Maximum power 63 kW at 6000 RPM
Maximum torque 110 Nm at 4100 RPM
Engine compression 9.5:1
Vehicle kerb weight 1060 - 1100 kg

Diesel Jimny:
Made in 2006
Engine Renault K9K 266, 1461 cm3
Maximum power 63 kW at 3750 RPM
Maximum torque 200 Nm at 1750 RPM
Engine compression 17.9:1
Vehicle kerb weight 1150 - 1180 kg

Information about transmission gearing ratios is present in this wiki article (although it is not 100% guaranteed that all the data there is correct).
Last edit: 08 Jul 2018 08:22 by Bosanek. Reason: Added a link

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08 Jul 2018 09:45 #194116 by OlaGB
And how about difference in accelleration and driving uphill?

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09 Jul 2018 19:33 #194147 by Bosanek
The major difference there is that the diesel vehicle can start from still and cruise along (even uphill) just on idle throttle. This is just impossible with the petrol vehicle - some throttle is always needed.
If the diesel vehicle is in 1st low or reverse low gear, it will start moving and go uphill on idle throttle no matter how steep it is. It's like a self propelling machine.

This is a major advantage (at least currently for me, who is accustomed to this) when maneuvering to (un)park on sloped streets or when tackling rocky off road terrain.

Apart from this, I did not take any precise measurements when driving uphill.
I did notice one general difference when driving on curvy roads uphill (like "switchback curves"). Usually you have to slow down on a sharp bend uphill, and lose engine RPMs there. When exiting the bend and applying throttle again, the diesel vehicle will happily "pull itself out" from a high-1000s RPM situation when throttle is applied (like from 1700 RPMs or so), while the petrol vehicle is hopelessly drowned even in low-2000s RPMs. You have to downshift with the petrol vehicle as soon as its RPMs drop to lower 2000s RPM range.
In other words, the diesel has no problems accelerating again when RPMs drop low (that's where its peak torque is), while the petrol forces you to keep the revs high or to downshift frequently on bends in order to maintain uphill speed.

I don't know if it's me being lazy with the matter of frequent gear shifting, but I find the behavior of the diesel vehicle much better suited for driving on mountainous roads where you just can't keep high engine RPMs (steady vehicle speed) all the time, because of the bends or blind spots.

Regarding straight-on acceleration from 0 to 100 km/h or so, I think they are similar, with the petrol possibly being a bit quicker. I did not measure.
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11 Jul 2018 20:30 #194203 by Busta

Bosanek wrote: P.S.:
The petrol engine did rev to much higher RPMs during the descends than the diesel engine (for example to 5500 compared to 3500), but that is incomparable anyway because of different engine construction.

I believe this is the bit that makes people think diesels have better engine braking. The engine revs don't rise as high so it feels slower, perhaps more controlled. This is the same affect that, in reverse, makes people think diesels accelerate quicker than equivalent petrols. The revs don't rise as quickly, which makes the sensation of accelerating more surprising.

Great experiment by the way, and very thoroughly executed!

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