Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

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LFY
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Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by LFY »

Hello to everyone.

I live in an area in which there is a relatively steep uphill. I have to pass it every time I want to drive my car. I have tested my car in that road and the results are as following:

1 - If I drive with 3rd gear, I need to stay @ 4500 RPM to travel at the speed of 80 km/h. I press throttle about 30% to keep a steady speed. If I press the gas pedal more, the car will increase speed.

2 - I also tried using 4th gear. With 4th I have to apply around 60% throttle and keep engine speed @ 3500 RPM to travel at 80 km/h. If I press the gas more than 60%, it accelerates and speed will rise.

I didn't try 5th gear because I thought it will be considered engine lugging.

Now my question is that which driving scenario is worse for my engine? I prefer to drive up that hill with 4th. Is it harmful for the engine to keep running with 60% throttle for about 15 minutes in 4th gear?

(1.8L 16V petrol engine with 5 speed manual transmission - Peugeot 406)

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Stickyfinger
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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by Stickyfinger »

3rd is not harmful but it is totally unnecessary. 4th gear is in the power band.

andy5
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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by andy5 »

It's most economical to drive with the engine speed around its best torque figure. This could be 1500 to 2200 rpm for a diesel, anything up to 4000 rpm for some petrols.

That said, if it isn't going to maintain speed like that, change down and have a higher engine speed. But that doesn't seem to be a problem for the hill you're discussing. Maybe for towing ...

I can remember driving a friend's Volvo with some sort of economy gauge in the dashboard, climbing on the M6 and some Scottish hills. Seemed to be best at 4000 rpm, which seemed foreign to me being more used to a diesel. I looked up the engine when I got home. Peak torque at 4000 rpm.

I tried looking up torque for a 1.8 Peugeot engine. There seems to be 2600 4000 and 4250 mentioned, depending on version You might try looking up your specific one, but from what you've said I'd guess 4000 might be it. Choose either 3rd or 4th, or maybe change between them occasionally where the steepness varies slightly.

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white exec
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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by white exec »

I don't get a lot of choice of gear when leaving home. Turn right out of the house towards of local village, and there's an immediate climb, rising 51m over 440m of single-track road (≅12%, 1-in-8) - which we concreted in 2010.
DSCF1677a.jpg
This is second gear all the way, no choice.

After that, it levels out a bit, with the next 0.9km rising 73m (so ≅8%, 1-in-12), but a good bit of it still curving dirt-track, so no chance of speed. Second gear all the way still.

At the top of the ridge (the yellow arrow), the track tips over a blind summit to a winding 2km sloping descent to the village, along concreted road, with only a brief part of it allowing 3rd gear.

So, 2nd gear gets plenty of use here, until you break out on to the tarmac'd two-lane mountain road that links Sayalonga village to coast. 6km of this, with just 300m offering a chance of 4th, so 3rd gear most of the way on this!
Sayalonga-Algarrobo.JPG
Thereafter, a decent 3km to the coast, with even some scope for 5th!

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by LFY »

Thanks for your great answers.
This is second gear all the way, no choice.
I think this is not bad because in the first 15 minutes after start-up, you need to drive slowly with low rpms until oil temperature goes up and you can rev the engine without worrying about wear and tear.
I tried looking up torque for a 1.8 Peugeot engine. There seems to be 2600 4000 and 4250 mentioned, depending on version
My car's engine is 1.8 16V (XU7JP4 LFY)
110 PS @ 5500 RPM
155 nm @ 4250 RPM
Redline and fuel cut-off = 6500 RPM

So, according to what you said, driving with 60% throttle in 4th will not ruin my engine. I don't care about fuel economy, I am just worried about engine health.

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by white exec »

LFY wrote:
26 Dec 2019, 20:47
This is second gear all the way, no choice.
I think this is not bad because in the first 15 minutes after start-up, you need to drive slowly with low rpms until oil temperature goes up and you can rev the engine without worrying about wear and tear.
Have the the advantage on the 2.5 diesel of the engine block/heater coolant being able to reach full working temperature within 2 mins of driving off: a very efficient twin-thermostat system.

Do try to be a bit considerate by allowing the engine to run for a couple of minutes when getting it out of the garage/driveway, before setting off up the hill. Half-a-km up it, the heater is producing decent warmth.

Set off and tun left, though, it's downhill all the way to the coast - around 4km - and while the XM temp gauge is up and about a bit more slowly, the RAV4's needle stays rooted lukewarm for most of that downhill run. That's the thing with diesels: they use minute amounts of fuel, and actually zero on the over-run. Only when they re-idle does the fuel start to be injected again. If it weren't for the heat of compression, downhill they'd run cold for ever!

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by Peter.N. »

As Andy says, the most efficient way to drive is around the point of maximum torque, I try to keep my diesels at around 2000 rpm, that's where you get best fuel economy.

Something I have only noticed recently, as my C5 has a continuous fuel consumption read out, is that going down hill in a lower gear to use engine braking often increases fuel consumption, unless its very steep. Its more economical on fuel to use a high gear that doesn't necessitate you using the throttle at all in your descent and brake when necessary. This obviously consumes a little brake lining but as I rarely use my brakes anyway I very rarely have to replace them.

Peter

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by LFY »

Peter.N. wrote:
27 Dec 2019, 13:06
As Andy says, the most efficient way to drive is around the point of maximum torque, I try to keep my diesels at around 2000 rpm, that's where you get best fuel economy.

Something I have only noticed recently, as my C5 has a continuous fuel consumption read out, is that going down hill in a lower gear to use engine braking often increases fuel consumption, unless its very steep. Its more economical on fuel to use a high gear that doesn't necessitate you using the throttle at all in your descent and brake when necessary. This obviously consumes a little brake lining but as I rarely use my brakes anyway I very rarely have to replace them.

Peter
What do you think about my first post in this topic regarding the pressure on my engine in 2 different driving scenarios?
Which one do you recommend?
Do you think driving with average RPM but more than 50% load will ruin my engine?
Is driving with 60% throttle in 4th @ 3500 RPM is considered engine lugging?
As I said earlier, fuel consumption is not my priority in this topic. I just want to know about what will happen to my engine.

Image

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by Peter.N. »

My experience with petrol engines is very limited, I haven't run one since the days of distributors and carburettors - and perhaps even dynamo's. :wink:

You shouldn't do any damage to an engine by driving it hard providing its within the normal rev range, thrashing an engine at high speed is counter productive as apart from the stresses involved, if you are much above above the maximum torque speed you are just wasting fuel with very little gain in power, especially with a diesel.

With most diesel engines, as already stated maximum torque is developed at around 2000 rpm and you are not going to do any harm by running it flat out at that speed, torque drops off rapidly above that speed so any increasing in fuelling will not produce an equivalent increase in power but will generate a lot of heat.

The story is not quite the same with a petrol engine as by nature they run more efficiently at higher revs and develop much less torque at low revs, driving at at a speed it sounds comfortable with would be my inclination, to get maximum efficiency that should be around the max torque figure. You should be able to tell from the sound of the engine whether or not you are over stressing it.

Most modern engines are pretty robust and providing they are not driven at ridiculous speeds they should be able to cope with full throttle operation, providing they don't overheat.

You wouldn't want to operate a petrol engine at very low speeds as it wouldn't perform very well, diesels on the other produce most of their power at low revs so if you drive it hard at below about 1500 rpm it could shorten the life of the DMF. You can't buy very high power modern diesels with a manual box because all that low speed torque tends to break them, automatic boxes are much stronger - although I wouldn't have one.

So, back to your original question, full throttle operation in the mid rev range would be the most efficient and least harmful use of your engine.

Peter

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by LFY »

Thanks for your great answer dear Peter!
You are a great moderator and a well-educated engineer!
I really enjoyed reading your post.
Now I'm sure that I don't ruin my car by passing that uphill every day at 3500 RPM.

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by Peter.N. »

Thank you. I don't do much moderating now, I'm 80, I think I am only being allowed to stay on as an honorary moderator. I'm pleased I was able to help.

Peter

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by ekjdm14 »

Just a small addendum to this thread, on the subject of engine lugging. Personally I'd only consider it "lugging" if you were above about 80% throttle in order to maintain your speed & increasing to full throttle yielded very little, if anything in the way of acceleration.

I think a good way to look at it is, if you're nearly flat out and not gaining speed it's time for a downshift or two. Petrol engines in general I change up at around 3000-3500 in normal driving, up towards 4500 if accelerating hard & I try not to let the revs drop below 2500 on a hill. Rather a generalisation I know, but it's worked for the last 20 years and I've not ruined an engine yet (and I have driven some on their very last legs this way without blowing them up too!)

To wander from the topic somewhat, modern diesels are much more varied in my experience. OK pretty much any mechanical injected diesel, turbo or not, is going to be happy anywhere between 1250-3000rpm no matter if it's a small TU based unit or a 4 litre Toyota L6. But the difference between the 1.4HDi and the 2.0 is night and day, the smaller engine is much more like a petrol car to drive & likes to rev a bit more whereas the 2.0 I found for normal acceleration I was shifting up at 2200-ish rpm until I was in 4th gear & then it'd be happy in 4th down below 25mph right up to 50+

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by Peter.N. »

I appreciate that some modern diesels seem to be designed for petrol like performance, others have very flat torque curves, but my comments were referring to run of the mill diesels like the Hdi and its derivatives.

Peter

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by Zelandeth »

Best example I've seen with this is in the van, which is a 2.8 tonne brick with 2.3 litres of normally aspirated diesel producing 78 screaming horses and somewhere around 100lbf of torque (can't remember the precise figure).

It's noticeable that when on the motorway if there's a long slight gradient involved that going for flat out throttle in top gear produces a much greater movement from the temperature gauge than dropping to fourth to maintain the same or similar road speed. It never moves much, but has always seemed to be an indication to me that the throttle position had a far greater impact on how hard the engine was working than crankshaft speed.

Torque and power curves have a huge part to play too...once up to speed it takes less throttle to maintain cruising at 70, and she will maintain that over slight gradients, whereas as 60 needs more throttle and will be knocked back by any grade or the slightest headwind, usually requiring a drop to 4th.

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Re: Which one puts engine under more pressure? High load lower RPM or low load higher RPM?

Unread post by white exec »

Zelandeth wrote:
04 Jan 2020, 10:57
. . . top gear produces a much greater movement from the temperature gauge than dropping to fourth to maintain the same or similar road speed. It never moves much, but has always seemed to be an indication to me that the throttle position had a far greater impact on how hard the engine was working than crankshaft speed.
I reckon that's one of the biggest clues: seeing that the temp gauge rises more rapidly in some circumstances than others.
The amount of fuel being injected/burned is largely proportional to how hard the accelerator is pressed, and this seems (in practice) to have a greater effect than increasing the number of injections/sec - i.e. dropping a gear.
In short, a labouring engine can use huge amounts of fuel. Particularly noticeable on a diesel, where using a lot of welly to climb a hill can produce a lot of smoke (even at the 'peak torque' revs figure), while dropping a gear and getting the revs up will not.