In your example of the two cars running side by side, true they are doing the same anount of work. But the larger engine, assuming it is more powerful, has the capability of doing more work, and this potential, the performance factor, seems to be achieved generally at the expense of fuel consumption.
If you take an extreme case, and put a socking great V8 into small car, everybody knows it will drink petrol even when tootling along at 30 mph. Isn't the answer that because it has the capability of doing 0-60 in 5 seconds, it must be running less efficiently when it isn't?
|
Cliff,
"If you take an extreme case, and put a socking great V8 into small car, everybody knows it will drink petrol even when tootling along at 30 mph."
Not exactly true.
My Camaro Z28 has the same engine as the previous model Corvette, the LT1 5.7L V8. As JBJ says, with the 4-speed auto, it's doing about 75mph at 2000RPM in top. It's not exactly a 'small car' but will still do the 0-60 in just over five and a half seconds. Average mpg is about 22 to 24, with 28 mpg obtained recently on a heavily congested return trip between Basingstoke and Sheffield. Over 30mpg is possible if you're not in a rush (or heavy traffic.) The 6 speed manual version is normally 1 to 2mpg better.
The belief that V8s (particularly American) are crude gas guzzlers is far from the truth nowadays.
The 3.2L XJ8 I have for work/commuting is pretty much identical to the Chevy in fuel consumption even though she's in non-sport mode.
Kevin...
|
|
In South Africa Ford used to sell two versions of the Cortina, one with a 1.6L engine, and one with a 3L engine (called the Perana). The Perana had lesser fuel consumption. This anomaly seems to be achieved when the engine matches the physical characteristics of the car and the way it is used. I think personally that larger engines have larger losses, hence cannot be considered like for like with smaller ones.
|
"larger engines have larger losses"
Other things being equal, perhaps, but lower revs reduce frictional and pumping losses - there's much to be said for big, lazy engines. Small engines with a high specific output can be terribly thirsty - the infamous Kawasaki 500cc H1 motorbike could consume a gallon in 20 miles!
|
|
|
hillman,
I think that the Perana was a 302ci V8 Capri, not the Cortina.
I owned three 3.0L V6 Cortinas when I lived in Namibia, including an 'Interceptor' for a few months. IIRC the basic engine for the Cortina was the 1.6 then the 2.0L. Next up was the 2.5 V6 (fitted to loads of bakkies) which was the most popular choice where I lived and the top of the range was the 3.0L.
You are correct that the V6s had very good fuel consumption. My MkIII 3.0GT averaged 32mpg on a trip to Etosha compared to 28mpg from the 2.0 Volvo that accompanied us.
Kevin...
|
|
|
|
Generally larger engines will tend to be the most efficient,very large marine diesel engines can achieve thermal efficiency's of around 50%. Compare this to the best efficiency a small DI high speed diesel engine can achieve of around 30%..Some 40 tonne trucks can average 10mpg.
The improved economy is largely due to the smaller surface area to volume ratio of each cylinder. As cylinder size increaese, there is less surface area for the given swept volume to lose heat to - so more heat is available to perform useful work. For this reason using a smaller bore and stroke and adding cylinder's to an engine results in more heat loss in addition to increased frictional losses and reduced efficiency.
Consider a petrol engine at tickover...unlike a diesel, all of the engine's useful work is used to overcome the friction within the engine (practically 0% efficiency). As throttle opening is increased, pumping losses reduce and efficiency increases.
Going back to the scenario of the two cars running side by side.
The 1.8 engine will have a greater throttle opening for a given road speed, so it will have the lowest pumping losses in addition to lower heat and frictional losses.
The 2.5 litre has the advantage of slower running which offsets some of the disadvantages allowing it to give comparble or slightly better economy than a smaller engine under high speed/load conditions.
|
Seems to me that since all engines tend to idle at about 800 rpm, and fuel-to-air ratio has to be kept pretty constant to satisfy the catalytic converter, it follows that a larger engine will consume more fuel when idling. If the vast majority of your journeys are spent inching through stop-start city traffic, for a given car, a bigger engine will prove more thirsty.
Cruising at a constant high speed requires a certain amount of effort to push that car through the air. That same amount of effort could be provided by a small engine with a wide throttle opening or a large engine with a small throttle opening. So if the vast majority of your travel is on a clear motorway (!) using cruise control, the larger-engined version of the same car may use no more fuel than the smaller-engined.
Turboed engines are food for thought - they have lower fundamental compression ratios, considered less efficient, but it doesn't alter the fact that for a given maximum power output they can have smaller swept volume. Consider the old Punto GT's turbo 1.4 producing 130+ bhp; I wonder how it fares against the similar-output 205 Gti's 1.9 or the Punto HGT's 1.8 for fuel economy during congersted city driving? Or indeed any periods when the turbo is providing little or no boost? During these periods it's still only drawing-in a 1.4's amount of fuel.
|
My V70 2.4T idles at exactly the 670 RPM mentioned in the owner's manual, though I don't know what the fuel consumption is because the instantaneous readout just says --- when the car is stationary. My Vectra GSi however used to switch from MPG to GPH when stationary, which I though was neat. When fully warmed up, stationary consumption flicked between 0.2 and 0.3 GPH, though I don't know how accurate this was.
Drive the GSi moderately hard, and the fuel consumption rose by a nominal amount over more gentle driving. Stoke the fire, and it rose by a bit more.
The turbocharged V70 2.4T is completely different. Driving gently or moderately hard behaves per the Vectra V6. Stoke the fire, and the consumption falls off a cliff!
As an aside, take a look at this link www.ussmissouri.com/speedVSFuel.aspx and scroll down to the graph at the bottom. It shows the fuel consumption of the USS Missouri (Under Siege, and all that) in relation to speed. Is there not a pronounced change at 16 knots, or what, and 10,500 gallons per hour at top whack?! Yikes!
|
SjB - >My V70 2.4T idles at exactly the 670 RPM mentioned in the owner's manual, though I don't know what the fuel consumption is when the car is stationary<. I think your car is trying to tell you that as it is covering 0.0 miles, you are getting 0.0mpg. Possibly very efficiently, but 0mpg nonetheless.
Presumably your Vectra would show about 2gph when travelling on the Mway?
|
Andrew T - The Vectra display switched from MPG to GPH below about 2 mph, so at least the display was still 'useful'. Intelligent design, I think. The Volvo display simply switches from showing MPG to displaying a series of dashes at low speed, so has 'no use'.
For what it's worth, a cold start in the Vectra showed .7 GPH, dropping to .2GPH by the time the engine had warmed up. Quite a graphic display of cold engines needing a richer mixture, even when not required to do any work other than turn themselves and ancillary devices such as the alternator and pumps, over.
|
"..dropping to .2GPH by the time the engine had warmed up"
Be very interesting to see that plotted graphically. How long are we talking about, and did this coincide with the temperature gauge's indication of 'warm'?
I try to run my car at tick-over for about a minute before setting off - this isn't enough to reach normal temperature, but at least it allows the oil to circulate and makes the engine feel a bit 'happier. To me, anyway.
I did have a colleague who insisted that engines needed to be loaded to warm up, so he took this to the extreme of driving off the instant it had started (sometimes even before the starter motor had finished its job!). He had a lot of trouble with his cars, but never seemed to make the connection...
|
|
|
|
Cruising at a constant high speed requires a certain amount of effort to push that car through the air. That same amount of effort could be provided by a small engine with a wide throttle opening or a large engine with a small throttle opening. So if the vast majority of your travel is on a clear motorway (!) using cruise control, the larger-engined version of the same car may use no more fuel than the smaller-engined.
Surely though the smaller engine, while being driven harder, will be operating more efficiently due to a wider throttle opening than the large car with little throttle, since the throttle losses are the most significant source of inefficiency in a petrol car?
Turboed engines are food for thought - they have lower fundamental compression ratios, considered less efficient, but it doesn't alter the fact that for a given maximum power output they can have smaller swept volume. Consider the old Punto GT's turbo 1.4 producing 130+ bhp; I wonder how it fares against the similar-output 205 Gti's 1.9 or the Punto HGT's 1.8 for fuel economy during congersted city driving? Or indeed any periods when the turbo is providing little or no boost? During these periods it's still only drawing-in a 1.4's amount of fuel.
The fuel economy on turbo cars i have driven has seemed much more sensitive to the way it is driven, as you suggest. The problem is that turbo cars are usually the sort that encourage some enthusiastic use of the throttle!
|
Is it true that the throttle losses are the single most significant source of inefficiency? (I'm not saying you're wrong).
In reality, the smaller engine will probably have shorter gearing. At a given road speed all the moving parts of the engine will be moving that much faster. Does that increase or decrease friction losses within the engine and gearbox? Surely the inertial loads on the crank and valvetrain will be higher when all those parts are reciprocating more quickly?
Someone send for LJKS !
|
|
I don't claim to know anything about frictional losses of fuel to air ratios etc, I can only go on real world experience. A couple years ago my uncle bought a new Toyota pickup equipped with a 2.4l 4 cyl engine. He had it for a little over a month before deciding it was too under-powered for his needs. Amazingly the dealer gave him all his money back against a new 3.0l V6 model of the same pickup. He found the V6 to give approximately the same MPG as the 4 cyl model, but neither of them bettered the MPG of my 5.7l V8 GMC which also happened to be a physically larger than his Toyotas.
|
|
|
|
Tunacat - apparently it is, or so i was taught. Hence one of the reasons that deisel engines, without a throttle, are more efficient.
|
|
|
|
Not for nothing is the Missouri at its most efficient at 16 knots, using 141 gallons per nautical mile, compared to 331 gpnm at 32 knots. (Yes, I was sad enough to copy the data into Excel to answer their question.)
But why does the fuel consumption graph have such a steep change at 16 knots? (It looks like 2 straight lines plotted together to me, so I don't for a moment imagine that it's accurate, but don't doubt that it will reflect real useage.) What is happening to the engine at this point?
I tell you, my little Polo flat out down the motorway wasn't very efficient. Perhaps more to do with its being built like a brick - but then so is the Mercedes.
|
But why does the fuel consumption graph have such a steep change at 16 knots?
At a guess, it could be that below this they ran with fewer boilers / engines. Many of our Navy's ships have 'cruise' and 'boost' engines for a similar effect. For example, on the Type 22s they have Tyne gas turbines as cruise engines that typically draw between 15 and 30 l/min and Olympus 'boost' engines which can go up to about 100 l/min. They only use the boost engines if they need to get somewhere in a hurry of course!
I would guess that the engines of the Missouri are maybe more efficient at high load so rather than running them all at lower load for lower speeds they only run a few.
|
|
|
|
mapm - As a very clumsy guess, I would assume the discontinuity in the graph will have something to do with Reynolds numbers and the point at which the craft starts to climb up its bow-wave?
|
Andrew-T - THANKYOU! :-)
When I made the post, I wanted to add a paragraph that fluid dynamics would play a large part, and tried hard to remember the term from my A level Engineering Science days that covered laminar, transitional, and turbulent flow.
Reynolds numbers. Of course!
Prompted by your prompt, using Google even gave me a website with a Reynolds number calculator, not that I need it in my current domestic or business (ERP computing) lives!
www.efunda.com/formulae/fluids/calc_reynolds.cfm
|
So how come, (unless this has been answered above and I missed it or didn't understand it!) a 14 litre diesel Scania hauling 40 tons can do about 10 mpg, my 2 litre diesel doesn't do 70mpg despite hauling only (very roughly) two thirds the weight per cc? In which case it should really, in proportion be doing about 100mpg.
And don't tell me the aerodynamics of a Berlingo are worse than a Scania (even if they are!)
|
|
|
|
|
|
|
|
