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See, I would have thought that a higher compression NA motor would have a little better efficiency than lower compression turbo motors at low engine loads. One reason why diesel engines have better economy is due to high compression ratios.
I also would have thought that smaller engines that have to work harder than larger engines to produce the same power output would be a little more efficient because more energy is being used to convert heat to motion. I thought that was the whole concept of a smaller engine in the Prius. But maybe it has more to do with fewer cyclinders and lower weight instead.
Turbo engines are only more fuel efficient than NA engine when under no load, which is what the scenarios are for testing purposes.
The moment one put the foot down the fuel usage goes way up. On boost they do make more power, and power takes fuel. On top of that, extra fuel is also pumped in to cool the charge, so even more wasteful.
If only governments around noted world use a more realistic testing scenario, like using perhaps more than 50% throttle, the trend will reverse and turbos will fall out of favour.
Most people in the real world do not use more than 50% of the throttle during daily driving, esp. with a torquey turbo engine.
For things like hydrocarbons high performance normally aspirated engines have some inherent disadvantages (high rev NA engines like cam overlap which hurts emissions, etc), but these are generally manageable at the expense of some performance.
CO2, which is driving much automaker decision making, is a different issue. It’s directly related to engine efficiency, so a more efficient (lower fuel consumption) engine will unavoidably do better. It’s this chase for efficiency that has driven the switch to turbo engines, but contrary to popular belief these are not only useful for gaming the test...
At peak efficiency there is very little to separate a well designed normally aspirated engine and a turbo, and in fact NA may have a slight advantage in typical high efficiency street applications (ie Prius). The main difference, however, is in where the efficiency is achieved- for a normally aspirated engine it’s typically at low to mid RPM and very high load. Friction at higher RPM and low loads saps efficiency, as seen in the BSFC graph (lower numbers are better):
Turbos, on the other hand, have a larger efficiency sweet spot that extends from low through medium-high load at low rpm. They are also happier sitting and responding at low rpm where there’s less friction:
Pondering these graphs a few things start to make sense. A Prius runs a normally aspirated engine because the hybrid system can keep the relatively small engine in the high load, low rpm sweet spot. A turbo engine driven like you stole it (at max torque/ power) will have economy as bad as any normally aspirated engine (or worse). A turbo engine driven at low load (as a performance engine typically will be on the street when driven normally) will have significantly higher efficiency than its normally aspirated counterpart if both have equal power.
So normally aspirated engines can be just as efficient if designed and used correctly. Unfortunately the recipe for that tends to be low average and peak revs and low overall power (to raise average load). As neither of these is ideal for a performance car the push has been towards turbocharging. The GT3’s engine, on the other hand, promotes a) using revs for drivability and power, b) a high overlap, gradual ramp rate cam, c) a relatively large displacement engine, increasing friction and thermal loss. Which goes a long way to explaining its dismal 13 mpg city while delivering only slightly more usable power vs the GTS.
So normally aspirated engines can be just as efficient if designed and used correctly. Unfortunately the recipe for that tends to be low average and peak revs and low overall power (to raise average load). As neither of these is ideal for a performance car the push has been towards turbocharging. The GT3’s engine, on the other hand, promotes a) using revs for drivability and power, b) a high overlap, gradual ramp rate cam, c) a relatively large displacement engine, increasing friction and thermal loss. Which goes a long way to explaining its dismal 13 mpg city while delivering only slightly more usable power vs the GTS.
Thanks for all this!! This is the sort of depth of information I was looking for and really helps put things in perspective.
I guess I thought that a really good engine making lots of power had to be very efficient in order to do so. If you're going to get 500 hp out of a 4.0L engine, you can't just go wasting gas to get there. You need to burn as much gas as efficiently as you can.
It didn't really occur to me that being able to burn a lot of gas efficiently doesn't necessarily mean you can also burn a little gas efficiently. I was quite shocked by the MPG numbers when they came out (and am not happy about paying the associated tax for them) and couldn't imagine how it was possible.
How can it burn more gas when driving miserly to move a 3k lb slippery sports car along than my 4k+ lb barn-shaped Wrangler Unlimited?? I thought maybe there was some accounting for "expected use" in the numbers or something but now it seems that's not the case. It's just kinda **** poor at just trundling along.
It's all smoke and mirrors. They're just better under test conditions and not everyday use. That's all that matters, meeting the "test criteria".
I have been saying this since the big VW emissions scandal. The turbo cars are the mechanical equivalent of cheating emissions tests and the mfr know it. For the testing conditions, the turbos will be more efficient. For real life driving, it could actually be the opposite for in town driving.
I have been saying this since the big VW emissions scandal. The turbo cars are the mechanical equivalent of cheating emissions tests and the mfr know it. For the testing conditions, the turbos will be more efficient. For real life driving, it could actually be the opposite for in town driving.
I can't agree. While the euro cycle and old EPA tests were very game-able, current EPA test does a decent job at estimating MPG for a car driven "normally". On that test the current GTS delivers over 32% more miles per gallon than a GT3 despite just a 10% power difference (and a speed difference that's far smaller still). If you're consistently using most of the power available MPG will obviously be poor in either case, but there are good reasons why turbos are not only good for test cycles. In my personal recent experience my 1M, which was more similar in weight and performance too its big brother than most realize, consistently achieved MPG E90 M3s could only dream of. As long as I wasn't "on it"...
There is clearly still some examples of poorly optimized turbos that can't outperform their NA counterparts. There's also some gaming that still goes on, especially for the European and Chinese markets. There are however far more examples of real improvements being made. I did some design of high MPG vehicles in a past life, and it's clear that unless you can go hybrid it's far easier to both MPG and performance targets with turbo. As much as I hate to admit it (I'll be keeping my GT3 for a long time I expect).
Speaking from experience only, my turbo cars have always used up more fuel per mile. The comparison that sticks in my memory is my Megane 2.0 turbo returned similar or worse mpg than my 997.2 C2S, which obvs had 3.8 NA. On paper the Porsche should have been worse but the reality is if you drive it like the sports car it is then the megane is on boost quite a lot and therefore sucking in shed loads of fuel.
It may be true that a turbo is more efficient for a given power output because it burns more of the fuel or it creates less waste heat (there are no other ways to be more efficient of course) but my guess is the difference is minor compared to how you drive.
My explanation for why I think turbos use slightly more fuel is due to the nature of the power delivery you are closer to 'on it' more of the time.
Also, the Prius uses a Atkinson Cycle, not Otto Cycle. In fact, when my RC F jumps into Atkinson mode on the highway @ 75 MPH, I can achieve ~31 MPG on flat highway. Not bad a for a 2 ton 5.0 V8.
I've owned a total of 12 Porsches since my first 2000 Boxster 2.7 and note the true everyday fuel economy of all..To avoid confusion i will ignore the diesels and hybrid Porsches i've owned and just compare the sportscars.All the NASP Porsches i've owned achieved virtually identical everyday real fuel figures to the UK official combined cycle figure from the first Boxster to my new 700 mile 991.2GT3.The only exception to this rule was my GT4 which driven in a normal manner only averaged 20% less than the official combined figure..Go figure that.!
The only turbo Porsche sportscar i've owned is my recently sold 991.2GTS coupe which averaged roughly the same as my GT4 (21 Mpg(UK) ) which is about 33% less than the combined figure..This applies to all turbo cars i've owned from other makers (BMW,Audi etc)..
From my extensive ownership of various cars the NASP are not necessarily more or less efficient than the turbo cars just closer to the official figures..Incidentally my new GT3 fuel economy is pretty similar to the GTS although i'm still limiting to the 5k rev running in.
01-30-2018, 09:03 PM
