A know it all neighbor declared that a V6 is the most efficient configuration. I know GM full size cars get amazing MPG. Brother in law's full size Buick w/V6 gets 34 mpg on the highway. I suspect that's partly due to fuel injection and probably a high rear end ratio. Is a V6 the most efficient layout?
V6 More Efficient?
- Thread starter borgward
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No, an inline 4 has the least amount of internal friction (per horsepower), technically. That said, the old GM Buick 3.8L V6 did very well with high rear end ratios like you said. There was once an MIT engineering effort to reduce ring friction in the Buick 3800, so that might have been part of it.
Are you talking about theoretical (carnot cycle) efficiency or fuel economy? The most efficient would be an undersized engine running really, really hot. A small engine driving a larger vehicle is possible, but of it might take a long time to accelerate up to freeway speeds. That's kind of what a gas-electric hybrid is supposed to do to improve fuel economy at freeway speeds. The engine is generally underpowered but can handle freeway cruise without the electric motor. Then the electric motor supplements the power from the internal combustion engine for acceleration and for slow speed driving.
A similar sized inline 6 might be more efficient than a V6 since it doesn't require balance shafts and counterweights.
A similar sized inline 6 might be more efficient than a V6 since it doesn't require balance shafts and counterweights.
For fuel efficiency the less swept volume the better, BSFC depends on hundreds of factors. V6 is not more efficient.
Given how many kinds of V6 engines there are, I would have a difficult time determining what engine design is ideal.
That in mind, there is one kind of V6 that I don't consider ideal for anything. That would be the kind of V6 which is basically a 90 degree V8 with 2 cylinders cut off. It is even worse when a balance shaft is added to compensate for the uneven design.
Another thing to consider is how turbochargers are installed. When you have a V6 engine, making it with a single turbo isn't ideal. Either you get 2 long similarly sized up-pipes such as the 80s Buicks and lose exhaust heat which causes turbo lag, or you can have some unbalanced design like the one found on the 1984-1989 Nissan 300ZX. Parallel twins are the ideal setup, but that is more costly. When you have an inline 6, having a single turbo is simple, and parallel twins are only used in special cars where people are prepared for a high price.
In 1992 to 1995, you could buy a Nissan 300ZX Twin Turbo or an LT1 Corvette to get 300 horsepower, yet the Corvette typically got better MPG.
That in mind, there is one kind of V6 that I don't consider ideal for anything. That would be the kind of V6 which is basically a 90 degree V8 with 2 cylinders cut off. It is even worse when a balance shaft is added to compensate for the uneven design.
Another thing to consider is how turbochargers are installed. When you have a V6 engine, making it with a single turbo isn't ideal. Either you get 2 long similarly sized up-pipes such as the 80s Buicks and lose exhaust heat which causes turbo lag, or you can have some unbalanced design like the one found on the 1984-1989 Nissan 300ZX. Parallel twins are the ideal setup, but that is more costly. When you have an inline 6, having a single turbo is simple, and parallel twins are only used in special cars where people are prepared for a high price.
In 1992 to 1995, you could buy a Nissan 300ZX Twin Turbo or an LT1 Corvette to get 300 horsepower, yet the Corvette typically got better MPG.
Originally Posted By: artificialist
Given how many kinds of V6 engines there are, I would have a difficult time determining what engine design is ideal.
You don't have to guess. Its been studied. Intuitive as well: The less friction surface area per engine displacement, the less internal engine friction you have. That means a 4 cylinder beats a 6.
Given how many kinds of V6 engines there are, I would have a difficult time determining what engine design is ideal.
You don't have to guess. Its been studied. Intuitive as well: The less friction surface area per engine displacement, the less internal engine friction you have. That means a 4 cylinder beats a 6.
Your "know it all neighbor" is not well informed as you might suspect.
A 4-cyl engine is more fuel efficient than a 6 cylinder engine.
A 3-cyl engine is more fuel efficient than a 4 cylinder engine.
This 3-cyl engine based on the geo metro's gets 84 mpg. It's not even a hybrid.
A 4-cyl engine is more fuel efficient than a 6 cylinder engine.
A 3-cyl engine is more fuel efficient than a 4 cylinder engine.
This 3-cyl engine based on the geo metro's gets 84 mpg. It's not even a hybrid.
There's way more to it than friction though--a longer stroke (with its greater friction) also has more torque for a given displacement.
I had a 98 Chevy Metro 3 cylinder, 5 speed and the absolute best mpg I ever got was 67 mpg. That was back highway steady 50-55 mph. Typical driving I would avg high 50's which at that point in my life was exactly what I needed. Now with 3 kids and wife it would not work.
Originally Posted By: AlaskaMike
There's way more to it than friction though--a longer stroke (with its greater friction) also has more torque for a given displacement.
The original question was if a V6 is the best choice for fuel efficiency. Just that one parameter, it being a v6 or not.
A V6 is burdened with more friction than an inline 4 or 3.
The highest efficiency engines are 4 or 3 cylinder, not 6.
There's way more to it than friction though--a longer stroke (with its greater friction) also has more torque for a given displacement.
The original question was if a V6 is the best choice for fuel efficiency. Just that one parameter, it being a v6 or not.
A V6 is burdened with more friction than an inline 4 or 3.
The highest efficiency engines are 4 or 3 cylinder, not 6.
V6s don't seem to be very fuel efficient in pickups/SUVs/vans. They are a compromise that costs less and might be easier to package than a V8, but offers more power than a naturally aspirated I4. It seems like they top out in the mid 20s for MPG in light truck applications and haven't improved much over the years. That's not to say they are bad engines, I have two V6 pickups, but I don't think efficiency is their strong point.
I would like to add that the managers at GM who decided to scrap the 3.8 over the cam chain eating fuel hog 3.5 L V6 deserve to be fired with real fire. Even with the fancy expensive 6+ speed transmissions they get worse mileage.
Rod
Rod
Originally Posted By: borgward
A know it all neighbor declared that a V6 is the most efficient configuration. I know GM full size cars get amazing MPG. Brother in law's full size Buick w/V6 gets 34 mpg on the highway. I suspect that's partly due to fuel injection and probably a high rear end ratio. Is a V6 the most efficient layout?
I wonder if he knows that an Inline 6 has been designated more efficient than a V6, that alone should disprove the know-it-all neighbor.
And we have not even begun to discuss forced induction, once the parasitic draw is overcome.
A know it all neighbor declared that a V6 is the most efficient configuration. I know GM full size cars get amazing MPG. Brother in law's full size Buick w/V6 gets 34 mpg on the highway. I suspect that's partly due to fuel injection and probably a high rear end ratio. Is a V6 the most efficient layout?
I wonder if he knows that an Inline 6 has been designated more efficient than a V6, that alone should disprove the know-it-all neighbor.
And we have not even begun to discuss forced induction, once the parasitic draw is overcome.
I think it was in the 19-teens that Alfa built a series of engines for GP racing. Trying to determine the most efficient design. The 4 cylinder consistently beat the others of similar displacement. They entered a huge 4 cylinder that it was claimed you could drop smaller engine block through the bore ... Can't recall if they won or not, but it set the stage for many years to follow.
I like a well balanced 3-cylinder. And OMC had great luck racing their 60 cu in 3-cylinder making upwards of 120 hp in full race trim. I've owned a number of three cylinder motorcycles and they were smooth. Whether they were inherently more efficient that a similar sized 4-cylinder I don't know ...
Usually the choice of layout comes down to packaging constraints... That's what gives us 5-cylinder Audi's and such
I like a well balanced 3-cylinder. And OMC had great luck racing their 60 cu in 3-cylinder making upwards of 120 hp in full race trim. I've owned a number of three cylinder motorcycles and they were smooth. Whether they were inherently more efficient that a similar sized 4-cylinder I don't know ...
Usually the choice of layout comes down to packaging constraints... That's what gives us 5-cylinder Audi's and such
The car around the engine has to be considered, too. A small inline-4 could get great fuel economy in a compact econobox. Put the same engine in a 3-ton SUV and it might do worse than a big V8. Even a bigger inline-4 wouldn't be a good idea because you'd get massive vibration above a certain displacement.
BrocLuno's comment about packaging constraints is crucial, too. If there's room for an inline-6, you might be able to squeeze a little more efficiency out of it vs. a V6 just because the better balance might allow certain engine parts (e.g. block, crank pulley) to be lighter. OTOH, if you're designing the car around the engine, you might do a little better with a V6 because the compact size allows the car around it to be lighter.
Everything is a system.
BrocLuno's comment about packaging constraints is crucial, too. If there's room for an inline-6, you might be able to squeeze a little more efficiency out of it vs. a V6 just because the better balance might allow certain engine parts (e.g. block, crank pulley) to be lighter. OTOH, if you're designing the car around the engine, you might do a little better with a V6 because the compact size allows the car around it to be lighter.
Everything is a system.
There is a lot of misinformation about engine efficiency and BSFC. Where is the engine supposed to be "more" efficient? Overall? At cruise speed? During stop n go city traffic?
The A, Number 1 factor affecting a conventional engine's BSFC is piston ring swept area. The lower the piston ring swept area, the more efficient the engine.
A 4 cyl eng with 350 cubic inches has far less piston ring swept area than a similarly sized V8, due to simple geometry. In addition, that 4 cylinder has far less combustion chamber surface area to transfer combustion heat away from doing useful work. The 4 cyl will achieve a peak BSFC of 0.42-0.45 Lb/HP/Hr, compared to the V8, which will achieve a peak BSFC of 0.52-0.55 Lb/HP/Hr.
Today, the Prius achieves a thermal efficiency of 40% (the most efficient production engine) , or a BSFC of about 0.36 Lb/HP/Hr due to various non standard operating modes and high rates of EGR flow.
Of course, a typical V6 has fewer main bearings than an inline 6. And a flat 4 has only 3 main bearings. Light aircraft often chose a flat, air-cooled, 4 cylinder of large displacement. The combo, designed in the 1930's is rather efficient, nearly matching the prius engine, with a peak BSFC of 0.38.
The A, Number 1 factor affecting a conventional engine's BSFC is piston ring swept area. The lower the piston ring swept area, the more efficient the engine.
A 4 cyl eng with 350 cubic inches has far less piston ring swept area than a similarly sized V8, due to simple geometry. In addition, that 4 cylinder has far less combustion chamber surface area to transfer combustion heat away from doing useful work. The 4 cyl will achieve a peak BSFC of 0.42-0.45 Lb/HP/Hr, compared to the V8, which will achieve a peak BSFC of 0.52-0.55 Lb/HP/Hr.
Today, the Prius achieves a thermal efficiency of 40% (the most efficient production engine) , or a BSFC of about 0.36 Lb/HP/Hr due to various non standard operating modes and high rates of EGR flow.
Of course, a typical V6 has fewer main bearings than an inline 6. And a flat 4 has only 3 main bearings. Light aircraft often chose a flat, air-cooled, 4 cylinder of large displacement. The combo, designed in the 1930's is rather efficient, nearly matching the prius engine, with a peak BSFC of 0.38.
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In completely general terms here - the V6s I've driven will effortlessly run the car down the highway in top gear while idling. 4 cylinders have trouble maintaining speed on a grade and will downshift. I'd think in this scenario the V6 would achieve better mileage.
34mpg from a full-size sedan in 2017 in the USA? Can tell you a certain European manufacturer's V12 was getting close to that figure in the 1990s.
The Inline-6 in my Euro gets up to 35.5mpg, that is only a tad worse than the factory claims - but this is A/C on at highway speeds. I've never been able to test A/C off on the highway - it gets uncomfortable.
At least in theory, a bigger engine shouldn't have to work very hard at all at highway speeds, whereas your typical 4-cylinder might be working harder.
As an anecdote, a friend's Daihatsu Sirion (a whopping 3cyl engine) when 3 passengers and two small dogs was constantly between 3rd and 4th at about 100km/h. 4th was enough mostly, but you'd have to plant your foot every so often to keep things going.
The Inline-6 in my Euro gets up to 35.5mpg, that is only a tad worse than the factory claims - but this is A/C on at highway speeds. I've never been able to test A/C off on the highway - it gets uncomfortable.
At least in theory, a bigger engine shouldn't have to work very hard at all at highway speeds, whereas your typical 4-cylinder might be working harder.
As an anecdote, a friend's Daihatsu Sirion (a whopping 3cyl engine) when 3 passengers and two small dogs was constantly between 3rd and 4th at about 100km/h. 4th was enough mostly, but you'd have to plant your foot every so often to keep things going.
Originally Posted By: d00df00d
The car around the engine has to be considered, too. A small inline-4 could get great fuel economy in a compact econobox. Put the same engine in a 3-ton SUV and it might do worse than a big V8. Even a bigger inline-4 wouldn't be a good idea because you'd get massive vibration above a certain displacement.
The question would then be.. what displacement would be too much? GM offers a 5.2L inline 4 diesel in some of their medium duty trucks. Have not heard of any issues with that.
The car around the engine has to be considered, too. A small inline-4 could get great fuel economy in a compact econobox. Put the same engine in a 3-ton SUV and it might do worse than a big V8. Even a bigger inline-4 wouldn't be a good idea because you'd get massive vibration above a certain displacement.
The question would then be.. what displacement would be too much? GM offers a 5.2L inline 4 diesel in some of their medium duty trucks. Have not heard of any issues with that.
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