Driven5 said:
In reply to DaewooOfDeath :
I have been enjoying working through this thought experiment, as this runs tangentially to numerous thoughts I've had, posted about, and continue to have.
Prius doesn't just use 'mild little camshafts' though, it uses a modified Atkinson cycle which trades power for efficiency. Adding forced induction basically turns that into a Miller cycle. Which is great, but doesn't seem to be what you're looking for either. It may not have VTEC (does have VCT though) or high flow ports, but are those really any more 'fancy' than forced induction?
If you're just talking about taking a ~1.5L economy engine, beefing up a few parts, and adding forced induction, pretty much every manufacturer has done that now and it works great. The big difference I can see from what you seem to be poking at is that it also eliminates the need to rev, which is actually a good thing for economical power production.
Running at 8k rpm is much less efficient than 4k rpm. For starters, there is a lot more friction. Some of this comes from things like the stiffer valve springs needed to rev that high too. Based on the BSFC and thermal efficiency charts I've seen, all seeming to peak in the 3k-4kish range, it seems likely that overall efficiency in general is highest at these speeds too... Which would make sense why economy engines are thus cammed for that range too.
So while you may be able to use lighter drivetrain components with the lower torque at higher rpm, there are even greater efficiency losses to be had up there as well. Ever seen the Top Gear where they run a Prius flat out vs an E90 M3 (at the Prius speed) in a fuel economy test? Spoiler: The M3 wins.
At low throttle and low rpm, the smallest and mildest engine wins the economy contest. However, as power needs increase, this doesn't hold true. If you have to run an engine above ~4k rpm to get the desired power, you're generally losing efficiency vs a more (peak) powerful engine running the same power in that sub-4k rpm range.
On to the forced induction. What you described for equal torque across the rev range is not something that a supercharger would be used to do. This is because even the smallest cammed naturally aspirated engines make peak torque in the mid-range rpms. So that would be your point of lowest boost pressure. You'd need greater boost both above AND below that point, with the more boost needed the further you are away. That's probably why ridiculously (artificially) flattened torque curves didn't start appearing until the advent of more advanced computer controlled turbo systems.
So now what you need is a turbo that can make significant boost at very low rpm, which means it will be less efficient on that engine at very high rpm. This is where you run into the excessive heating of the intake charge to maintain the desired output.
This is very interesting. Thank you for writing it all out. There are a three quasi educated guesses I'm making that might be mistaken.
The first is that drivetrains capable of handling high loads eat up considerably more power than lighter drive trains. I am basing this on the differences between engine dynos and chassis dynos for the same engines. The extreme case is with trophy trucks. It's not uncommon for an 800 hp motor on an engine dyno to be around 500 wheel horsepower in a trophy truck. I don't know how those losses are divided, but I'm assuming a TH400 capable of handling both 600 lb ft and the massive shock loads associated with jumping (ie, constantly gaining and losing traction) is a major component of that loss. I compare this with something like a stock Miata, where the drivetrain loss is usually somewhere in the high 20 or low 30 hp range.
I realize tires are a part of this as well, but we're still talking about a Miata losing 270 less horsepower between the engine and the road. The assumption I'm making here, that might not be realistic, is that it's not just a flat percentage of power lost in the drivetrain, but something closer to an absolute number. Ie, it takes a lot of power just to turn the drivetrain in a trophy truck, and that if we put a Miata 1.6L in that trophy truck and connected it to that beefy TH400, the truck would barely move. As with your Prius top speed example, that 1.6L trophy truck, constantly struggling to reach its 34 mile per hour top speed, might also get worse fuel economy than the 800 hp monster simply because it would always be working so hard.
On the other hand, assuming the drivetrain survived long enough to make a dyno pull, that 800 hp trophy truck engine installed in a Miata would make something like 700 wheel horsepower, not 500. I am extrapolating that to mean that, from an OEM point of view or from the point of view of an enthusiast who wants to exceed 4 miles per gallon, we should try very hard NOT to require a drivetrain that can survive 600 lb ft and high shock-loads.
The second assumption I'm making is that a T5, rated at 300 lb ft of torque, can be made very small and very light, while a comparable drivetrain rated at 600 lb ft of torque, like a TKO, needs to be much heavier, much larger, and much more expensive. In the specific cases of the T5 and the TKO, the TKO is about double the weight, around quadruple the price, and about 30% larger. Avoiding the need for a TKO, it seems, is thus a significant cost and packaging benefit, along with a bunch of smaller perks like T5s being nicer to shift.
Third, I'm assuming that those BSFC numbers you reference (and that I've seen as well), are more a consequence of engines being small than engines being inherently efficient at 3-4k rpm. Ultra efficiency optimized ICE engines, I'm thinking busses and ships, tend to operate at the lowest RPMs they can get away with. My assumption was therefore that we don't optimize 1.5L economy car engines for 1200 rpm peak torque because that engine would struggle to make 30 hp, not because it wouldn't be efficient to do so. We cam them to make power at 3-4K rpm because gets us close to 100 hp, which is enough, and a 1.5L engine losing efficiency at those higher rpms isn't enough to offset the benefits of being small displacement. Ie, if we can get 40% BSFC from a 100 hp, 4 liter engine at 1200 rpm, or 35% BSFC from a 100 hp, 1.5 liter engine at 4,000 rpm, the 1.5 is going to get you better fuel economy inspite of being slightly less efficient in terms of BSFC.
I need to stress, these are all assumptions I'm drawing from inference. I don't have a laboratory full of equipment, and I certainly haven't tested any of these assumptions myself.
