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Small engines on the motorway

Discussion in 'off topic' started by ciderglider, Mar 7, 2013.

  1. ciderglider

    ciderglider pfm Member

    The accepted wisdom seems to be that small engines are econmical when pottering around town, but that at speed they're no more economical (maybe less so) than a larger engine. But I've not found any hard evidence for this, just vague claims that a smaller engine will be "working harder" at speed than its larger brethren. So, what's the truth?
  2. Steven Toy

    Steven Toy L3 Toy

    It depends largely on gear ratios. Powerful engines with lots of torque can work well with gear ratios well under 1:1 so at 80 mph the car is barely doing 2000 rpm. A car with a smaller, less powerful engine may only have its highest gear at direct drive (1:1) so will be doing well over 3000 rpm at 80 and consequently will use more fuel.
  3. JTC

    JTC PFM Villager...

    Not very scientific, but i recall a Top Gear test between an M3 and a Prius showing that the M3 was more economical doing circuits than was the Prius.
  4. Darth Vader

    Darth Vader From the Dark Side

    As ever 'it depends'. The shape of the car is important as at speed the wind resistance increases. My mini Cooper was very bad for wind resistance at around 100mph. BTW it was legal for me to drive as fast as I liked on a motorway all those years ago. The M6 was MT on a Sunday! The 70mph limit was brought in during an oil crisis and has stayed ever since.

    So a small engine with little HP and a bad body shape would struggle however my Cooper S was I think only 1200cc but could reach 120mph or so the clock said.


  5. Eyebroughty

    Eyebroughty JohnC

    I used to have a Cooper S, now that was a fun car :D
  6. Darth Vader

    Darth Vader From the Dark Side

    I used to take mine on the nightmare rally. It started at midnight and finished by dawn. The last time we got trapped down a lane behind an overturned car.


  7. cutting42

    cutting42 Heading to Fish Hacker Erg \o/

    I suspect there is not much in it. A small engine will nearly always be more economical than a similar technology large engine. However I remember Top Gear drove a Prius round their track flat out and got 17 mpg. They drove an M3 round at the same pace and the M3 got 19mpg so there might be some truth to it.

    Go to 3min 30sec
    Last edited by a moderator: Mar 21, 2018
  8. Eyebroughty

    Eyebroughty JohnC

    I used to think I could drive quite fast. One day a chap I knew, who did a lot of rallying, took me out in his rally tuned Cooper S.

    When I got out the car at the end of what he described as "a quick spin" he thanked me for trying to reshape his footwell. :eek:
  9. Richard Nichola

    Richard Nichola pfm Member

    This is the same Top Gear that pretended a Volt had broken down when it hadn't. Also hurtling around a test track is hardly a good replica of either motorway or town driving.
    Last edited by a moderator: Mar 21, 2018
  10. Anh

    Anh Naim ghetto blaster

    An engine's efficiency and b.s.f.c varies depending on RPM and load.

    Ideally you want an engine to be at its most efficient RPM at motorway speeds to get the best MPG.

    I would take a larger engine over a smaller engine based on it's effortlessness and smooth ness of power delivery, small 4 banger engines can be fatiguing, more so than a Naim CDX.
  11. andyoz

    andyoz pfm Member

    Car showrooms from about 2015 onwards are going to look very sci-fi indeed.

    The shift to lightweight composites and hybrid engines, driven by the energy prices shock about 3 years ago, is nearly there. It takes about 5 years for manufacturers to respond to major shifts in the market.

    Who wants to keep seeing (nearly) triple digits appearing on the petrol pump when filling up.
  12. sean99

    sean99 pfm Member

    I think town/urban mpgs are mostly a function of vehicle weight (since it's being constantly stopped and started) and motorway mpgs are mostly a function of aerodynamics.

    Large engines can penalize around town because the mass of moving components in the engine is much larger and must be accelerated each time the car accelerates. Also more cylinders = more engine drag.

    I suspect it's the fact that smaller engines run at higher RPMs on the motorway than larger ones that offset their better efficiency, and I have found that small (1.2, 1.4) engines seem to lose much of their efficiency advantage when sitting on the freeway for a couple of hours. The small engines probably don't develop enough torque at their optimum efficiency RPM so have to be revved higher than ideal.

    Oh, and Top Gear are a bunch of idiots, Clarkson especially. A bunch of permanent adolescents.
  13. Rodrat

    Rodrat pfm Member

    Didn't the Volt run out of Volts rather than break down?
  14. Tony Lockhart

    Tony Lockhart Avoiding Stress, at Every Opportunity

    Many years ago I had a 1.3 Nova SR. It had quite high gearing for the engine size, and with just me in it, driving down to Lincoln from Leeming to see my girlfriend, at the end of the month so no money, I could easily prise 55mpg out of it. The car weighed not a lot, was fairly aerodynamic, and did from memory 25mph/1,000rpm. Also, I didn't need to waste fuel by braking for the roundabouts then accelerating again.

    If you're talking apples compared to apples, I think most of us would choose a two litre saloon over an eighteen hundred, as we'd know that in practice there'd be very little in potential economy, but the slightly larger engine would deliver us to our destination in a more relaxed state.

    Adding cylinders is what really kills economy, even with cylinders being 'shut down' when cruising. That's partly why you so rarely see small V6 or 8 engines.....

  15. tones

    tones Tones deaf

    There was a Times (I think) test comparing the fuel consumptions of a Prius and a 5-Series BMW diesel on a run from London to Geneva, and the Beemer won.

    We have a 1994 1.6L petrol Astra Caravan, and it seems to give its best consumption on the Autobahn - in one run to Vienna, we nearly reached the magic 50mpg. I think it has an especially good Autobahn cruising gear, which helps a lot.
  16. cutting42

    cutting42 Heading to Fish Hacker Erg \o/

    No of course not but it does illustrate the point that a big engine "can" be more efficient in certain circumstances.

    Technology plays a part as well. The M3 engine is extremely efficient despite being a performance engine. The Toyota engine is nothing special, it is a mass market regular petrol engine designed to work at its best at low revs and with the support of the electric motor.
  17. tones

    tones Tones deaf

    We've only recently been able to receive it in our Alpine fastness. What was reasonably entertaining in YouTube clips, I found, as you say, rather silly. However, given its worldwide popularity, there are a lot of silly people around.
  18. Anh

    Anh Naim ghetto blaster

    I seriously doubt the M3 returned 19 MPG on a track, more like 5-10 MPG.
  19. cutting42

    cutting42 Heading to Fish Hacker Erg \o/

    Watch the vid, it was just following the Prius around matching its speed. If it was being driven flat out I agree, much higher consumption.
  20. cutting42

    cutting42 Heading to Fish Hacker Erg \o/

    I am not surprised, the Prius running without its electric motor is quite poor actually, beaten by many efficient burn petrol and even more diesels. The BMW diesels are remarkable. I have had 2 and driven many more with even the 3.0l engine returning mpg in the high 40's with the 2.0l up at 55-60 whilst retaining excellent performance. A colleagues Prius on the same London to Manchester drive in heavy traffic returned 45 mpg where my 320d got 58!

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