Which petrol engine will have better MPG? - Viktorkaleb

Which petrol engine will have better MPG when driving on the motorway - between a small engine that runs with 70 mph at 3,500 rpm and a larger one that runs with the same speed but with 2000 rpm? Hypothetically, the conditions are the same. The only difference is the engine.

Which petrol engine will have better MPG? - badbusdriver

The question isn't relevant because the small engine will be in a small car, but in order to get a petrol which pulls 2000rpm at 70mph, it would have to be a bigger car so isn't going to be comparable.

Pretty sure all of this has been gone over before, but just to reiterate, it is how far open the throttle which determines the mpg, not specifically the revs.

But given how little of your weekly commute is at motorway speeds, I can't see the savings being more than marginal at best.

Which petrol engine will have better MPG? - Andrew-T

The question isn't relevant because the small engine will be in a small car, but in order to get a petrol which pulls 2000rpm at 70mph, it would have to be a bigger car so isn't going to be comparable.

Traditional wisdom used to be that to optimise fuel consumption a car should be driven near the peak of the torque curve or a little higher, before wind and rolling resistance increased too much. Most diesels peaked around 2000rpm, petrols about 3000 - roughly.

So a petrol running at 2000rpm, whatever the road speed, may not give the ideal solution.

Which petrol engine will have better MPG? - skidpan

Traditional wisdom used to be that to optimise fuel consumption a car should be driven near the peak of the torque curve or a little higher, before wind and rolling resistance increased too much. Most diesels peaked around 2000rpm, petrols about 3000 - roughly.

So a petrol running at 2000rpm, whatever the road speed, may not give the ideal solution.

Not on modern turbo petrols. The VAG 1.4 TSi has peak torque (184 lbs ft) available from 1500 to 3500 rpm. Even the 1.0 TSi in our Fabia has its peak torque (148 lbs ft) available from 2000 to 3500 rpm.

Its what makes them such a pleasure to drive compared to non-turbo petrols. The last 2 examples of that type of car I drove were a Toyota Avensis Tourer 1.8 (test drive) and a Honda Civic 1.8 we had in the office as a pool car. Both had decent bhp (think it was 145 ish for both - more than the VAG 1.4 TSi) but both had their max torque (probably about 140 lbs ft) at about 4500 - 5000 rpm. Rev the nuts off them and they went well enough but drive them as I prefer to drive a car and they were slugs with no go at all.

Which petrol engine will have better MPG? - Andrew-T

<< Not on modern turbo petrols. The VAG 1.4 TSi has peak torque (184 lbs ft) available from 1500 to 3500 rpm. Even the 1.0 TSi in our Fabia has its peak torque (148 lbs ft) available from 2000 to 3500 rpm. >>

I have no doubt that is correct. But how does fuel consumption vary while the management system is making peak torque available right across the rpm spectrum ? There must still be a sweet spot somewhere.

Which petrol engine will have better MPG? - RT

<< Not on modern turbo petrols. The VAG 1.4 TSi has peak torque (184 lbs ft) available from 1500 to 3500 rpm. Even the 1.0 TSi in our Fabia has its peak torque (148 lbs ft) available from 2000 to 3500 rpm. >>

I have no doubt that is correct. But how does fuel consumption vary while the management system is making peak torque available right across the rpm spectrum ? There must still be a sweet spot somewhere.

In this context, petrols are no different to diesels - when manufacturers quoted a range of engine rpm for maximum torque, it means that the natural torque peak has been electronically clipped to produce a flat torque curve - ususally to protect the transmission.

My car, a VW 3.0 V6 TDi, has quoted maximum torque from 1,750-2,500 rpm but a bit of mathematical curve fitting shows the natural peak would be at about 2,000 rpm. But whilst my engine's sweet spot is at 2,000 rpm it gives better consumption by going slower due to aerodynamics so I treat 1,750 rpm in 8th gear as the sweet spot - that gives 70 mph.

Edited by RT on 02/05/2021 at 07:56

Which petrol engine will have better MPG? - Andrew-T

<< In this context, petrols are no different to diesels - when manufacturers quoted a range of engine rpm for maximum torque, it means that the natural torque peak has been electronically clipped to produce a flat torque curve - usually to protect the transmission. >>

Ah, clever stuff. So we are seeing a truncated peak torque, not what the engine is theoretically capable of. And maybe to protect the turbo as well as the transmission ?

Which petrol engine will have better MPG? - skidpan

<< In this context, petrols are no different to diesels - when manufacturers quoted a range of engine rpm for maximum torque, it means that the natural torque peak has been electronically clipped to produce a flat torque curve - usually to protect the transmission. >>

Ah, clever stuff. So we are seeing a truncated peak torque, not what the engine is theoretically capable of. And maybe to protect the turbo as well as the transmission ?

In the case of the 1.4 TSi the 184 lbs ft limit (250nM) was to protect the DSG transmission versions which has a 250 NM limit in the dry clutch versions. The manual transmission was actually capable of transmitting more but it was a commercial decision to have both manual and auto exactly the same engine spec.

Seat did a 1.8 TSi in the Mk3 for a few years but it was not a big seller. More expensive, less economical and despite having 190 PS it still had the 184 lb ft of torque quoted for the 1.4 but over a slightly wider range, think it was 1500 to 4000 rpm. On the road the car was not a great deal quicker unless you thrashed it all the time. The reason was the transmission, Seat used the dry clutch DSG as the 1.4 TSi which as I said above has a 250 NM limit (184 ft lbs) thus it had to be protected.

The big issue with the 1.8 TSi was actually the buyers. Most saw it as a very cheap Golf GTi once re-mapped but the power and torque that was easily liberated greatly exceeded the capabilities of the transmission (especially the dry clutch DSG) withe the expected consequences. On the Seat forum the people doing this did not worry about the fact they could destroy their car since it was under warranty and when asked about the next owner all they could say was "its their problem then". Nice people. Main reason why I left the Seat forum.

Which petrol engine will have better MPG? - Andrew-T

In carburettor days (before ECU management) the bigger the engine, the more fuel it used - all else being equal - but not by much. I am thinking of Pug 205s, which had 1.0, 1.1, 1.4 or 1.6 engines. Essentially the same car, but its weight increased a bit with each engine size, so that probably explained some of the difference. Cars with bigger engines also get fatter tyres, which will add to rolling resistance.

In theory it should require the same energy to overcome wind and rolling resistance, so if all engines are set up correctly the fuel consumption should be similar. Variations will come with how the driver uses the throttle to make progress.

I would guess an ECU system should limit the differences between engine sizes, but I am certainly no expert. If the car is able to cruise happily at motorway speed (most are these days) it will be up to you to optimise consumption.

Which petrol engine will have better MPG? - badbusdriver

In theory it should require the same energy to overcome wind and rolling resistance, so if all engines are set up correctly the fuel consumption should be similar. Variations will come with how the driver uses the throttle to make progress.

Exactly my point re throttle opening in both this thread and the OP's previous thread (where I'd hoped we sorted out the obsession with rpm above all else!).

But in order to get a petrol engine which would pull 2000rpm at 70mph, I'd guess you'd be looking at a 2.0 turbo or a n/a of at least 2.5. Which unless you go for a hot hatch, realistically means something like an Audi A4 or BMW 3 Series.

Yes you could go down the diesel route, but given the £5k budget (from the previous thread) and the 100 miles per week spent on a motorway, I doubt the benefits would overcome the potential risks should something go wrong on a modern(ish) diesel. This would almost certainly wipe out (and more) any savings you made over a smallish n/a petrol such as the (very reliable) 1.25 engined Fiesta.

Which petrol engine will have better MPG? - madf

In theory it should require the same energy to overcome wind and rolling resistance, so if all engines are set up correctly the fuel consumption should be similar. Variations will come with how the driver uses the throttle to make progress.

Exactly my point re throttle opening in both this thread and the OP's previous thread (where I'd hoped we sorted out the obsession with rpm above all else!).

But in order to get a petrol engine which would pull 2000rpm at 70mph, I'd guess you'd be looking at a 2.0 turbo or a n/a of at least 2.5. Which unless you go for a hot hatch, realistically means something like an Audi A4 or BMW 3 Series.

Yes you could go down the diesel route, but given the £5k budget (from the previous thread) and the 100 miles per week spent on a motorway, I doubt the benefits would overcome the potential risks should something go wrong on a modern(ish) diesel. This would almost certainly wipe out (and more) any savings you made over a smallish n/a petrol such as the (very reliable) 1.25 engined Fiesta.

My 1.3L CVT Honda Jazz pulls 2200rpm at 70mph. It is a 2012 model so the comments are a decade out of date,

Which petrol engine will have better MPG? - catsdad

Viktor If you want to estimate fuel consumption across a range of cars you can look at the Real MPG section on the tab at the top of the HJ homepage.

If you are thinking about changing the Fiesta mentioned in your other thread you need to work out if the cost of changing is worth it for what are probably small savings in fuel cost. For example my current car does 55 mpg, my last one did 45. At 10000 miles a year the ballpark savings in my new car are only about £240.........65p a day.

If the Fiesta is reliable changing it to save a small amount on fuel is not worth it.

Which petrol engine will have better MPG? - skidpan

We have a 1.0 Fabia TSi 110 PS and at 70 mph it does about 2300 rpm. On a coastal run it will average about 60 mpg.

The Superb 1.4 TSi 150 PS did about 2200 rpm at 70 mph and on the same rum would average about 54 mpg.

But the Superb is much bigger and heavier however its also more aerodynamic. Put the Superb engine in the Fabia and it may be better, put the Fabia engine in the Superb and it would definitely be worse. My personal feeling is the engines are perfectly suited to those cars.

When we changed the Superb I was considering a 2.0 TSi Superb which does nearer the "magic" 2000 rpm at 70 mph which for some reason you seem to desire. The mpg reports in test put me off the car, touring seemed to be in the low to mid 40's.

For your own sanity forget the rpm at 70 mph. Manufacturers test for many 1000's of miles to get the correct combination of drivability and economy. I am sure that Skoda could fit the Fabia with gearing to match your magic number but it would only be on the flattest (or downhill of motorways where you could use top gear and even then you would need a downchange for safe lane changes in traffic.

We have had one diesel which did under the magic 2000 rpm at 70 and that was a BMW 2 litre, it was actually about 1900 rpm. The car had 143 PS and it pulled fine and would average mis 50's on a coastal run. At the same time we had a Kia Ceed 1.6 CRDi 115 PS and was doing about 2200 rpm at 70 mph. On the same coastal run it would also do the mid 50's. The BMW was much quicker but the trip time was virtually the same. Big difference was the overall mpg while we owned the cars, the BMW averaged about 47 mpg, the Kia 51 mpg.

Just buy what you fancy and enjoy it and remember that years ago cars did not have rev counters. My first 5 cars did not have one but since there was a space in the binnacle for one on that 5th car I bought one to make the dash look complete, the plain black dummy "instrument" looked plain daft.

Which petrol engine will have better MPG? - Terry W

My Pug 308 auto is doing a little over 2000 rpm at 70mph. 1.2L, 130bhp. So I suspect that there are many cars of modest size and capacity which do less than 2500rpm.

Back to the original question - is a large or small engine more economical at a given speed. As the energy consumed is similar (same body, same aerodynamics, same weight (+/-) it should make no difference.

However internal design of the engine will almost certainly be different - frictional losses from different bearing sizes, different number of cylinders, different induction and exhaust layout, possibly developed at different times etc could make a difference.

Whether this makes the larger or smaller capacity engine better is difficult to judge!

Which petrol engine will have better MPG? - badbusdriver

My 1.3L CVT Honda Jazz pulls 2200rpm at 70mph. It is a 2012 model so the comments are a decade out of date,

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My Pug 308 auto is doing a little over 2000 rpm at 70mph. 1.2L, 130bhp. So I suspect that there are many cars of modest size and capacity which do less than 2500rpm.

The OP is asking about cars doing 2000rpm at 70mph, not a little more. His current and previous cars were both manual, no mention of auto.

This is of particular relevance for madf's comments, who's car, (like the 2017 Jazz CVT) we used to own, has a CVT7 transmission. While this does not have gears as such, the CVT is programmed to have 7 'steps' to mimic having seven gears, like a modern t/c auto or DCT with that amount of gears (plus, in a Jazz CVT at 70mph, the merest hint of a hill, or even a headwind, will send the revs up to at least 3000). The manual Jazz has five gears, and in 5th gear at 70mph will be pulling close to 3000rpm.

T/C auto's are also usually pulling lower revs than the manual version as the torque converter allows this. I don't know what the actual rpm of the manual version of your car at 70mph, but I'd be surprised if it wasn't higher than 'a little over 2000rpm'(?). Also, with the increase in problems being reported on the 1.2 Puretech, I'm not sure I'd be willing to take the risk changing from a car known to be reliable, to one which may not be reliable, for the sake of a few pounds saved in petrol over the course of a week.

Which petrol engine will have better MPG? - Engineer Andy

In carburettor days (before ECU management) the bigger the engine, the more fuel it used - all else being equal - but not by much. I am thinking of Pug 205s, which had 1.0, 1.1, 1.4 or 1.6 engines. Essentially the same car, but its weight increased a bit with each engine size, so that probably explained some of the difference. Cars with bigger engines also get fatter tyres, which will add to rolling resistance.

In theory it should require the same energy to overcome wind and rolling resistance, so if all engines are set up correctly the fuel consumption should be similar. Variations will come with how the driver uses the throttle to make progress.

I would guess an ECU system should limit the differences between engine sizes, but I am certainly no expert. If the car is able to cruise happily at motorway speed (most are these days) it will be up to you to optimise consumption.

The rolling and air resistance of a car is just as important (the second more so at higher speeds as it varies with the square of the velocity) as weight or engine configuration. A high-sided car will do far worse at higher speeds (compared to itself at lower speeds) than a more sleekly designed, but heavier car.

Obviously it depends on how fast they are driven and the actual difference in weight to be able to determine if and when the latter becomes more fuel efficient at high speed. I'd suspect that the chances of a big but sleek barge overtaking the mpg of a similar year designed small supermini or city car being close to zero, as long as they were of the same engine type (can't really compare a petrol with a diesel).

The tyres will certainly make some difference, but only depending on the type/make/model and wear, not speed. The Aero resistance becomes a more important factor as speed increases, hence why for rapid in-car cooling, it is often recommended to use open windows up to around 40mph and A/C above that speed, as air resistance dramatically increases.

Which petrol engine will have better MPG? - SLO76

Which petrol engine will have better MPG when driving on the motorway - between a small engine that runs with 70 mph at 3,500 rpm and a larger one that runs with the same speed but with 2000 rpm? Hypothetically, the conditions are the same. The only difference is the engine.

Not quite as simple as this. There’s plenty of other variables such as weight and drag coefficient plus gearing to factor in plus some manufacturers are just better on fuel than others but assuming an identical car but with two different engines one larger but with taller gearing it’s very likely that the bigger one will be better on fuel at higher speeds but worse around town as it has to work harder to overcome the taller gearing. Best example I can think on was the Mk III Cavalier which was marginally better on fuel in 1.8 form than the 1.4 as it was spinning at substantially lower revs at speed.
Which petrol engine will have better MPG? - Big John

Which petrol engine will have better MPG when driving on the motorway - between a small engine that runs with 70 mph at 3,500 rpm and a larger one that runs with the same speed but with 2000 rpm? Hypothetically, the conditions are the same. The only difference is the engine.

Hmm, my dad's Ford Zodiac 3.0 v6 had lower revs on a motorway in overdrive(for the day!) - but was truly awful re mpg. Saying that it had the streamlining of an old barge.

Fast forward 40 years or so and my small 1.4 engined but turbo rather large Skoda Superb runs with 70mph at under 2400 - which is nicely inside it's wide peak torque band. At this speed mpg is outstanding - if you manage to cruise @70mph your average mpg would be nicely over 50mpg.

Edited by Big John on 03/05/2021 at 19:52

 

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