OEMs and Fuel Economy

I might be completely wrong here, but it seems that the cars that get the best real world fuel economy in class, whatever that class is, tend to have two things in common almost all of the time and another two things that seem to hold most of the time.

Seem to be always true:

1. A low cylinder count compared to the competition. A 944 is a good example here - 30 mpg when its similarly performing 6 and 8 cylinder rivals were getting high teens.

2. Low to moderate revs. Nothing that is designed for 8500 rpm gets good mileage compared to its rivals, as far as I can tell. V-Tech Hondas are partial exceptions, but not really.

Stuff that seems to be true most of the time:

1. Fewer camshafts. Pushrod and SOHC inline engines seem to have sizeable advantages over similarly performing rivals.

2. Naturally aspirated. I know you can supposedly supercharge for fuel mileage, but I've never seen it work in real life. The economy turbos seem to me an EPA cycle trick more often than not.

(edit) Stuff that doesn't seem to matter at all:

1. Displacement, as long as you aren't getting above about 750 cc/cylinder.

2. Valvetrain layout, with the possible exception of DOHC, which seems to be a little worse.

3. Whether or not the engine is percieved as "high tech."

So, if I'm right, why don't we all drive SOHC 3.0L four cylinder cars when we need 220 hp? Why don't we have 1.4L SOHC twins when we need 100 hp? Why is the DOHC high revving, small displacement V8 a thing? Basically, why aren't all the V shaped engines like the Buick 3.8 or Chevy LS engines and why aren't all the inline engines like H series Hondas?

I'm not sure that I agree with all of that.

Sometimes a bigger engine gets better real world economy. For example: gas engined work trucks. The only 6 cyl powered work trucks that I've ever had that didn't get worse mileage than a v8 was a '96 e250 with the last of the 300ci straight sixes. I've also had a Ford 3.8 v6 powered van and a GM 4.3 powered van, both had to be at Full throttle working thier buts off to go anywhere. We had a couple of 351 powered Econolines in the fleet too, they generally returned a mpg or two more from a gallon and were easier to drive because they weren't at WOT all day.

Currently I have a 4.8 LS powered Chevy work van. It does OK, but the one long wheelbase van that we have has a 5.3 and gets better mpg while hauling more weight.

All of the DOHC turbo SAABs that I've had got quite respectable mpgs for the weight and power of the car when I could resist going in for gratuitous boost pressure.

I've observed pretty much everything you mentioned. In my experience, large displacement (within reason) really doesn't have much of an effect on mpg. Cylinder count, on the other hand ... the 300ci I6 is kind of a case in point.

Oh I dunno. I've always felt redblock Volvos - SOHC, low-revving 4 cylinders - don't get particularly good gas mileage compared to larger-engined peers.

The newer turbo motors are an odd one. They have a much wider mpg spread than an NA motor depending on how it's driven. Drive it gently, it'll get good mpg. Start pushing it a bit and the super-rich OEM tunes start dumping fuel to keep the motor alive in the hands of idiots, so the gas gauge drops like a rock. Heck, watch a newer turbo BMW at WOT. They're usually rich enough to have visible exhaust smoke...

bastomatic wrote: Oh I dunno. I've always felt redblock Volvos - SOHC, low-revving 4 cylinders - don't get particularly good gas mileage compared to larger-engined peers.

I just ninja edited the original to say something I should have said originally. Displacement under about 750 cc/cylinder doesn't seem to matter much. There doesn't seem to be any fuel efficiency for a 3 liter v8 over a 6 liter v8, for example.

Compared to contemporary designs, are Volvos that bad? I thought they were pretty old designs.

rslifkin wrote: The newer turbo motors are an odd one. They have a much wider mpg spread than an NA motor depending on how it's driven. Drive it gently, it'll get good mpg. Start pushing it a bit and the super-rich OEM tunes start dumping fuel to keep the motor alive in the hands of idiots, so the gas gauge drops like a rock. Heck, watch a newer turbo BMW at WOT. They're usually rich enough to have visible exhaust smoke...

Seems like that's an attempt to make turbos and 11:1 compression co-exist. That's another thing I'm kind of wondering about. How much of an advantage is that compression? My old DSM with AWD and 7.8:1 compression got high 20s on the highway.

In reply to DaewooOfDeath:

The redblocks are indeed old designs - I think the OHC version started in 1976? I usually got about 18-22 in mine. In comparison, I usually got about 18-22 in my much larger Caprice with a 305 V8.

DaewooOfDeath wrote: Seems like that's an attempt to make turbos and 11:1 compression co-exist. That's another thing I'm kind of wondering about. How much of an advantage is that compression? My old DSM with AWD and 7.8:1 compression got high 20s on the highway.

The higher compression helps a little with mpg, but it helps a lot with low end torque when you're not in boost yet. And it helps get the turbos spooled faster. Between turbo improvements, higher compression, etc. the difference in lag between an 80s factory turbo setup and a new one is incredible.

DaewooOfDeath wrote:

rslifkin wrote: The newer turbo motors are an odd one. They have a much wider mpg spread than an NA motor depending on how it's driven. Drive it gently, it'll get good mpg. Start pushing it a bit and the super-rich OEM tunes start dumping fuel to keep the motor alive in the hands of idiots, so the gas gauge drops like a rock. Heck, watch a newer turbo BMW at WOT. They're usually rich enough to have visible exhaust smoke...

Seems like that's an attempt to make turbos and 11:1 compression co-exist. That's another thing I'm kind of wondering about. How much of an advantage is that compression? My old DSM with AWD and 7.8:1 compression got high 20s on the highway.

Compression ratio increase is more thermal efficiency. 8 to 11 is a very big jump- but 8-10 would also generate most of the thermal efficiency gains.

Your old DSM car probably would have been in the mid to upper 30's with a more modern version of the engine. More if the goal wasn't power, but fuel economy.

A lot depends on how hard the engine has to work.

Which is why you get less mpg towing than not.

Huge cylinder volumes are efficient (better chamber volume : chamber area ratio than more smaller cylinders), but they have a negative effect on NVH. From a vehicle NVH standpoint, we should all be driving 2 liter V12s.

For mileage we should all be driving 2.0L Thumpers :)

Revs - Cruising RPM seems to make a much bigger difference than the RPM the motor is capable of.

I don't think the peak BSFC of ICE engines has gotten that much better over the past 25 years, but the size off the sweet spot has grown a ton.

ProDarwin wrote: Huge cylinder volumes are efficient (better chamber volume : chamber area ratio than more smaller cylinders), but they have a negative effect on NVH. From a vehicle NVH standpoint, we should all be driving 2 liter V12s.

To a point- for gas engines, the bigger the chamber size, the long it takes to burn, and the farther from the ideal otto cycle you get. Which is why there's an upper limit on gas engine sizes, but not so much for diesel.

But for cars ant most trucks, that's a pretty good generalization.

ProDarwin wrote: Cruising RPM seems to make a much bigger difference than the RPM the motor is capable of

Yeah, pumping losses are a bitch. Ideally, if we had super awesome CVTs and other tricks, cruising speed would be at the lowest practical RPM while staying just below the power enrichment threshold to get the lowest pumping losses possible before richening up the AFR.

When you apply more "throttle" to climb a hill, it would raise the revs and increase throttle to keep manifold pressure just below the enrichment threshold, but at a higher rpm to deliver more power (up to the point where you'll have to start enriching to get the desired power).

Lean burn helps too by de-powering the engine a bit so you're running higher manifold pressure at the same rpm and power output, but it comes at the expense of higher NOx emissions.

rslifkin wrote:

ProDarwin wrote: Cruising RPM seems to make a much bigger difference than the RPM the motor is capable of

Yeah, pumping losses are a bitch. Ideally, if we had super awesome CVTs and other tricks, cruising speed would be at the lowest practical RPM while staying just below the power enrichment threshold to get the lowest pumping losses possible before richening up the AFR.

The Prius has such a CVT... but it doesn't have the same pumping losses issue due to its "Atkinson" cycle.

rslifkin wrote:

ProDarwin wrote: Cruising RPM seems to make a much bigger difference than the RPM the motor is capable of

Yeah, pumping losses are a bitch. Ideally, if we had super awesome CVTs and other tricks, cruising speed would be at the lowest practical RPM while staying just below the power enrichment threshold to get the lowest pumping losses possible before richening up the AFR. When you apply more "throttle" to climb a hill, it would raise the revs and increase throttle to keep manifold pressure just below the enrichment threshold, but at a higher rpm to deliver more power (up to the point where you'll have to start enriching to get the desired power). Lean burn helps too by de-powering the engine a bit so you're running higher manifold pressure at the same rpm and power output, but it comes at the expense of higher NOx emissions.

I have always wondered this.... is it more efficient generally to climb a hill by downshifting and running higher RPMs with less "load" (if that's the right term), or to stay in a higher gear as long as possible and put your foot down harder as long as you can still maintain forward motion ?
or perhaps due to engine mapping, AFR, etc... on different engines, maybe it's too hard to make any generalizations there ?

WRT turbos, supers and naturally aspirated economy - we've done extensive studies of the same engine in all three configurations On the highway, a turbo Miata engine will get slightly better economy than a naturally aspirated one, and a supercharged one will be slightly worse. I think it all comes down to pumping losses, the turbo is still moving air and decreasing any losses through the intake. The supercharger is sucking back a bit of power and offers a greater restriction even with the bypass.

With the advent of hugely adjustable cam timing, you can effectively adjust the compression ratio of the engine. This lets you run efficient high compression out of boost and keep the engine alive when you get into it. OEMs also run very different AFRs than the aftermarket - the gap between what the OEMs are doing and what the aftermarket is doing is opening up. Modern engines and engine management are getting really, really complex.

lastsnare wrote: I have always wondered this.... is it more efficient generally to climb a hill by downshifting and running higher RPMs with less "load" (if that's the right term), or to stay in a higher gear as long as possible and put your foot down harder as long as you can still maintain forward motion ? or perhaps due to engine mapping, AFR, etc... on different engines, maybe it's too hard to make any generalizations there ?

Up to the point of fuel enrichment, it's usually more efficient to be in the taller gear at lower rpm. Once you get into fuel enrichment, it's usually time to downshift.

Using my Jeep doing 60 mph on the highway as an example, the onboard display shows better mpg climbing mild hills in 4th (~1800 rpm) vs 3rd (~2600 rpm). If you make the hill steeper to the point where it's really working and well into fuel enrichment climbing in 4th, then dropping to 3rd has no noticeable effect on fuel burn, but it's easier on the engine (as it's not under as much load and won't be in fuel enrichment).

lastsnare wrote: I have always wondered this.... is it more efficient generally to climb a hill by downshifting and running higher RPMs with less "load" (if that's the right term), or to stay in a higher gear as long as possible and put your foot down harder as long as you can still maintain forward motion ?

In theoretical land, its always more efficient to just put your foot down more.

In real world, as you increase throttle the car will run richer, go open loop, change cam timing, ignition timing, etc. depending on how it is tuned. It may not be better.

rslifkin wrote:

lastsnare wrote: I have always wondered this.... is it more efficient generally to climb a hill by downshifting and running higher RPMs with less "load" (if that's the right term), or to stay in a higher gear as long as possible and put your foot down harder as long as you can still maintain forward motion ? or perhaps due to engine mapping, AFR, etc... on different engines, maybe it's too hard to make any generalizations there ?

Up to the point of fuel enrichment, it's usually more efficient to be in the taller gear at lower rpm. Once you get into fuel enrichment, it's usually time to downshift. Using my Jeep doing 60 mph on the highway as an example, the onboard display shows better mpg climbing mild hills in 4th (~1800 rpm) vs 3rd (~2600 rpm). If you make the hill steeper to the point where it's really working and well into fuel enrichment climbing in 4th, then dropping to 3rd has no noticeable effect on fuel burn, but it's easier on the engine (as it's not under as much load and won't be in fuel enrichment).

Generally for one engine, that may be right.

But applying that to all engines isn't. Some engines, the best BSFC is at 3000 rpm, some it's 1800 rpm. It does tend to be higher loads, but also to a limit.

So to answer the original question- other than looking at a specific engine's Brake Spec Fuel Map, it's hard to know without trying.

Bear in mind- pumping losses can be partially dealt with via cam timing and EGR. And gains can be lost quickly due to knock or temperature constraints.

Really- it's the reason there so many different designs and ideas to get fuel economy. VVT, VTEC, EGR, turbos, DI, specific cam events (to trim the style of combustion cycle), even IF there's a throttle (BMW).

So far, nobody has THE answer. Just a lot of good ideas. And many of them work really well.

one of my friends has a 99 Corvette with the LS1/6 speed as his daily driver and he claims an average of 34mpg on the tanks where he doesn't beat on it at all, which is better than the 31mpg that i managed a couple of times in my 01 GTP 4 door with the supercharged 3800 and right at where the first couple of tanks of my current 99 Saturn SL1 with he SOHC 1.9 backed with a 5 speed got before i started hypermiling it and taping off most of the front of the car to get it up closer to 40mpg..

One cool thing about modern engines - the throttle pedal isn't a throttle pedal. It's a torque request. The engine then does what needs to be done to deliver that torque. Cam timing, ignition timing, throttle plate position, assist from a hybrid system, whatever. I know the Skyactiv engines are messing around with the throttle opening a lot more than you think, manipulating things like fuel useage and cat temperature.

novaderrik wrote: one of my friends has a 99 Corvette with the LS1/6 speed as his daily driver and he claims an average of 34mpg on the tanks where he doesn't beat on it at all, which is better than the 31mpg that i managed a couple of times in my 01 GTP 4 door with the supercharged 3800 and right at where the first couple of tanks of my current 99 Saturn SL1 with he SOHC 1.9 backed with a 5 speed got before i started hypermiling it and taping off most of the front of the car to get it up closer to 40mpg..

The LS engines are great on the highway, but thirsty around town. I swear they use more fuel at idle than they do at 60 mph.

Keith Tanner wrote:

novaderrik wrote: one of my friends has a 99 Corvette with the LS1/6 speed as his daily driver and he claims an average of 34mpg on the tanks where he doesn't beat on it at all, which is better than the 31mpg that i managed a couple of times in my 01 GTP 4 door with the supercharged 3800 and right at where the first couple of tanks of my current 99 Saturn SL1 with he SOHC 1.9 backed with a 5 speed got before i started hypermiling it and taping off most of the front of the car to get it up closer to 40mpg..

The LS engines are great on the highway, but thirsty around town. I swear they use more fuel at idle than they do at 60 mph.

My 13B has an idle injector duty cycle of about 5% and a 60mph duty cycle of about 6-8%. I typically see 20-23mpg highway and deep into the single digits city driving. My drive to work has a lot more city and I've gone through five gallons in 35 miles.

After experiencing what a high compression low-pressure turbo can do for economy, I'm a believer. It's not so much there for more power as it is for energy recovery at part throttle and a little bit of extra oomph when needed because, really, 1.9l for a 3200lb car is silly.

Keith Tanner wrote: The LS engines are great on the highway, but thirsty around town. I swear they use more fuel at idle than they do at 60 mph.

There's a reason for that. Big engine does have a higher idle fuel consumption than a small engine. A lot of stock LS powered cars were also not terribly light, but have decent aerodynamics and tall highway gearing. So it takes some grunt to get it up to speed from a stop. Add that with the high idle consumption and boom, E36 M3ty fuel economy around town.

On the highway, with good aero, you're not using a ton of power and are likely in a more efficient operating range than idle, so it's probably not burning a whole lot more fuel, and thus, it ends up being fairly efficient.