What goes into making an efficient engine?

[Ecky]

Earlier I was pondering on why the new 5.0L Mustang gets equivalent gas mileage to a Honda S2000, while having a better power to weight ratio. This is a pretty extreme example, but why does the 5L 400hp Ford have similar FE to the 2L Honda?

I imagine Honda gave their F20C/F22C engines a very short stroke (robbing them of efficiency) to allow them to rev high and produce such impressive power figures in a relatively light package, but I'd have thought the bigger Ford engine would have significantly more pumping losses due to the large displacement, plus the Honda engine has a higher compression ratio.

What other design sacrifices did Honda have to make? What goes into making one engine more efficient than another, aside from stroke, compression, and displacement? What kind of magic did Ford work?

[serialk11r]

The F20C has decent efficiency actually, lowest BSFC is 242g/kWh which is really quite good when you consider that a newer 2GR-FSE with greater per cylinder displacement and direct injection has a peak "below 230g/kWh" (according to Toyota). I wouldn't read too much into stroke and stuff, on production engines that's not going to be the thing that decides fuel economy at all.

The problem is the transmission gear ratios, if you're comparing it to a Mustang. The Mustang has a "suggested" 1-4 skip shift in the manual transmission which really helps on the EPA cycle. The S2000 has very bad gears for fuel economy, its 60mph cruise speed is well into the 3000s. Despite all that, you'll probably notice that Mustang 5.0 owners are typically getting lower fuel economy than S2000 owners.

In general sports cars with fast acceleration suffer on the EPA test by default because the EPA test has standardized acceleration/deceleration, which requires them to run their engines at very low outputs (see Lotus Elise). Manual transmissions usually get shafted because the EPA test has a predetermined shift schedule that is bad for fuel economy compared to the typical automatic shift logic, and because manufacturers assume manual transmission buyers want a "sporty" experience and shorten all the gears.

TLDR: don't trust the EPA ratings too much, and back in the day when no one gave a crap about fuel economy they didn't have proper cruising gears in the transmission.

[pete c]

I just don't get manufacturer gear ratio selections. There is absolutely no reason for a 6 speed manual tranny to not have a very tall 6th gear, good for cruising only. Actually, the same applies for a 5 speed as well. 4 closely spaced ratios are plenty for any sort of racing. This is especially true with todays variable valve timing engines that pull smoothly from just off idle to redline.

My '09 Sonata is a good example of this. It is a 5 speed four cylinder. And it is the furthest thing from a sports sedan, yet hyundai decided to equip it with 5 close ratios anyhow. At 70 mph, it is getting pretty close to 3 grand. First gear is the opposite. It is just about the fussiest car I have ever driven to launch smoothly. Takes way too much concentration. I would have equipped it with much wider ratios. A lower first and a higher 5th. I really don't need a car that will cruise smoothly in top gear at one half the speed I typically run it at.

And then there is the current auto tranny cog battle. Mercedes has a....what is it, this week, 8 speed? I believe somebody has a 9 speed in the works. Yeah, that 6 liter v-8 is kinda peaky (insert rolleyes emoticon here).

For autos, i really believe anything beyond 5 or 6 cogs is just showing off.

[chrisoverson]

Surely if they need to change gears that often a CVT would be better? Or would this be an issue due to the fact some drivers don't like steady RPM while accelerating...

I'd imagine there are a lot of factors to consider. Efficiect engines should automatically adjust timing to maximum possible advancement by use of a knock sensor... use EGR correctly to reduce manifold vacuum... use different measures to ensure the engine heats up faster during cold starts etc. Also such things as higher compression, gearing for low RPM while cruising as said above, variable valve timing, smarter turbo control on turbo models (e.g. variable vane turbos that can adjust for power or economy on demand).

What else can I think of... cylinder deactivation as VW is now implementing, injector shutoff during engine overrun, automatic stop/start, regenerative braking, smart alternators that adjust their output, lean burn modes, heat recovery systems to make use of wasted exhaust heat.

There are so many factors really, but when it comes to the manufacturers the problem is they must consider multiple types of drivers when making a car. They don't tend to design cars just for people who drive slowly, smoothly and with greatest efficiency (with the exception of some hybrids maybe) but they need to cater for anybody who might drive, including those who want to get some power out of it.

Also it depends on the car it has to go in. If you have a light car with great aerodynamics then you can use smaller displacements. If it's going in a sporty car or a boxy/heavy car, then it needs more torque. Basically, if you design a car JUST for efficiency, it would be very different than a lot of them you see today. It's always a compromise between power, efficiency, aesthetics, and cost.

[pete c]

Chris,

Yes, design does need to be a compromise and if we still had 3 speed manual trannies, gear selection would have to be a compromise. But now every manual I am aware of has at least 5 speeds. Any of these vehicles have a 4th gear capable of 'go directly to jail' speeds. So, why not make that 5th gear higher? Is a wider gap going to affect performance? I seriously doubt it.

[chrisoverson]

I agree totally! Don't get me wrong, my comment was referring to the 8 speed gearbox by mercedes

I myself want to lower my 5th gear, although I must say the gearing is pretty good in mine. 2nd gear gets me to 70, and 3rd gear takes me anywhere from 0 to 90mph... so acelleration is not really an issue for me, but 5th gear at 60mph sits me nicely at 2700RPM... which isn't too bad for cruising in a 1.8 petrol.

I've been wanting to put in the diesel gearbox, or maybe even just give mine the diesel 5th gear (both the same box just different ratios), as it's 14% lower.

I believe these days all cars should have low RPM in 5th gear, using only the other gears for acceleration and 5th as a cruising gear. Or, like the Honda Civic diesel that had a 6th gear for cruising at something rediculous like 1500RPM.

[97hxcvt]

hi im new to the forum world... but like u were saying about the cvts. i have a 97 hx 203k with the cvt and ive been doing a general avg of 45+ mpg. i just cant believe how back in 97 they can get efficency like this that lasts. and yet it seems as if the latest and greatest are screaming look at me estimated 30 or 40mpg, but then my dad says his diesel chevette he bought brand new would get 50mpg but no power of course...

[ProDarwin]

As said above, Japanese gearing My S2000 turns 4200rpm at 80mph.

American 6 speeds, especially in large v8 cars, generally have two overdrives. The 6th gear is usually crazy-low cruising gear.

[GRU]

At highway speeds the mustang will probably be around 1500 rpm where he honda is probably around 3000 rpm.

[Ecky]

Makes sense!

My Del Sol suffers from this as well. It's a 5 speed but at 80mph the engine turns at 4300rpm. I'm going to swap out the tranny soon for the longest B-series transmission that came in a production car, which would still put me at over 3500rpm at 80, or 2900rpm at 65mph.

Where did you find the BSFC for the F20C?