The effect of horsepower on gas mileage

[Ted Hart]

Originally Posted by Flatland2D

Horsepower is also a function of rpm, so it's possible to have high torque and low horsepower.

IF HP=(T x RPM) / 5252 ... let's assume we want the HP at 5252 RPM...
Shazam! The 5252 cancels out the 5252, and we have HP=TORQUE! This points out one concrete point : HP sells cars , torque moves cars!
Torque is from classical physics (a force applied through a lever arm), while horsepower is derived from a typical horse turning an imagined turnstyle at some constant speed (RPM) for a measured time. Quite unscientific! And crude!And quite old! Give me torque anyday! -whitevette

[GasSavers_maxc]

Originally Posted by Ted Hart

IF HP=(T x RPM) / 5252 ... let's assume we want the HP at 5252 RPM...
Shazam! The 5252 cancels out the 5252, and we have HP=TORQUE! This points out one concrete point : HP sells cars , torque moves cars!
Torque is from classical physics (a force applied through a lever arm), while horsepower is derived from a typical horse turning an imagined turnstyle at some constant speed (RPM) for a measured time. Quite unscientific! And crude!And quite old! Give me torque anyday! -whitevette

Given to cars. One with 400hp,500Ft torque. One with 450hp,450ft torque.They both weight the same traction best gearing. More
hp wins 1/4 mile. In theory.

[omgwtfbyobbq]

Torque is a function of how much fuel is being combusted and some efficiency, and horsepower is torque at some rpm. Gasoline engine efficiency is, among other things, a function of displacement. A corvette and Insight are not that far off in terms of CdA and weight, but the vette get's crap for mileage due to it's large displacement engine having much more in the way of pumping losses at some given output.

Generally, manufacturers have increased power by increasing displacement, which increases pumping losses at some given output and reduces efficiency. This is why GM adopted cylinder deactivation. It allows increased engine efficiency via decreased pumping losses at some output, while still having the other four cylinders available for peak power. At maximum output a gasoline is only 10-25% less efficient than a diesel, but since the large majority of use is at less than maximum output, efficiency suffers due to pumping losses.

This is why Pulse and Glide is so effective for the Prius. By accelerating up to some speed, the driver is effectively minimizing pumping losses by maximizing output. Faster acceleration implies more fuel, and more importantly, more air, which increases the pressure in the cylinder during the intake stroke and reduces the negative work done by the pressure difference between the cylinder and crankcase. Anyway, by breaking up driving into an acceleration phase, and engine off deceleration phase, with the high/low speed window centered on some average speed, the driver will see an increase in FE compared to using the cruise control at the same average speed.

Now, a question I have is. Why don't hybrids just have 30hp engines with battery packs that allow for ~40-50 miles all electric? They'd get something like 80-100mpg in normal driving, with all the kickass acceleration an electric motor can provide. Well, I think this has to do with the manufacturer's unfamiliarity with battery packs and electric motors, they know ICE motors, and they may have heard that electric motors and battery packs can last for so long, but they don't have the long term empirical data to back this up. Preformance concerns are also present, with only 30hp, the car wouldn't be able to cruise at 90-100mph. They also have to deal with contracts that are years in advance, so they can't up and cancel all those ICE engines they've already ordered, which is why GM is in such a tight pickle. Their ~7 year development cycle is still geared for SUV's in a world where gas will probably never go below $2 a gallon, so they have to wiggle around with their parts suppliers and manufacturing such that they can sell as much as possible, and loose as little as possible.

In any event, given Toyota's short ~3 year development cycle, they should be set to roll out a Prius with more power via supercharging, a bigger, plug-in li-ion battery pack, and ~100mpg in 2008. Now, the only problem with this Prius is that on gasoline, not counting plug-in, it won't show as much of a difference when utilizing Pulse and Glide, but for the average driver, the FE will be much better.

Now that I mentioned it, turbocharging allows for the efficiency of a small engine, and the power of a larger one. It does this by reducing pumping losses at some load via the energy present in the exhaust gasses. A CAI will theoretically increase pumping losses a little bit by bringing in colder air that will decrease cylinder pressure, which is why a WAI increases efficiency slightly. But these are very small changes. Diesels are efficient because they don't suffer from pumping losses since they're compression ignition. They rely on the overlean running condition, and have no throttle plate to restrict air into the cylinder. Otoh, if a diesel ever gets down to stoich, it would probably melt because there would be gobs of fuel the didn't totaly combust and would melt through the rings.

Anyway, **** I wrote a lot. Turbocharging your engine probably won't result in any increase in FE, however, replacing your current engine with a smaller displacement turbocharged version with the same peak power will. If you toss a turbo on, you'll still need Xhp to move at Nmph, and the engine will still bring in the same amount of air, so you won't see a decrease in pumping losses. ITBs can increase hp and FE by allowing you to lean out your mixture, which reduces pumping losses and improves efficiency, but a standalone ecu can do the same much more accurately. A taller transmission/taller tires/bigger rims results in the engine turning slower at the same speed, which means it'll need more air/fuel to make the same amount of power, so that can help a bit. If you toss a 200-250hp PDI diesel with different maps in an Insight, you can probably get something like 80-100mpg EPA highway with lots of hp/tq. It'd probably make a really nice swap in a rabbit too, but the Cd's too high and mpg would be in the mid 60's. Unless aero mods really helped a rabbit down to the Insight's .25, then you'd have sports car performance with great FE.

[white90crxhf]

Originally Posted by Jonathan_ED3

I believe it's more a function of the individual characteristics of the HF engine.

I'm sure ECU tuning plays a large part, though.

i'd say it's the engine, with my si transmission i cruse at 65mph with 3k on the tach, i'm getting rougly the same gas mileage as before and my car will actually move when i put my foot down.

[GasSavers_Randy]

Originally Posted by omgwtfbyobbq

A corvette and Insight are not that far off in terms of CdA and weight, but the vette get's crap for mileage due to it's large displacement engine having much more in the way of pumping losses at some given output.

Vettes are more than 3000 lbs; Insights are less than 2000. You can get 30mpg in a Vette... Chevy fans love comparing them to S2000's when someone says Hondas are fuel-efficient.

Diesel mileage isn't really fair because there's about 12% more energy per gallon. Fuel costs per mile is fair enough, and BTU per mile is fair too, but straight MPG with different fuels is silly. You couldn't compare things like CNG, and ethanol or LPG would be tragic. The lack of a throttle, high compression ratio, and the way those combine with a turbo do make diesels really efficient motors, just 12% less efficient than you think.

There seems to be a lot in common between efficiency and horsepower, but it's down to the individual thing. Like a rich mixture bumps up power, but obviously trashes mileage. Things like indexing plugs or high compression helps both.

I don't know if individual throttle bodies would help mileage or not, but I haven't heard of them used for mileage. They seem to be on things like motorcycles and performance v12's. The big thing being the great throttle response. Tuning is per-cylinder though, so it would be a big pain.

[omgwtfbyobbq]

Originally Posted by Randy

Vettes are more than 3000 lbs; Insights are less than 2000. You can get 30mpg in a Vette... Chevy fans love comparing them to S2000's when someone says Hondas are fuel-efficient.

Yes, but it can't get 50mpg, even though the vette glider is only marginally more inefficient than a insight glider. 1000lbs~500kg only adds ~10N of force to overcome rolling friction, whereas the force needed to overcome fluid friction at that speed is ~200N. Weight does not have that much of a bearing on FE, especially highway FE. A vette glider would be getting ~50mpg EPA highway compared to an insight glider if they had the same engine, but the 6.0L engine results in way too much energy lost when not using the ~400hp available. Go check out the FE ratings for the 2007 Malibu on fueleconomy.gov. They are all the same car, all have automatic 4sp transmissions, but as the engine size increases FE decreases because pumping losses increase.

Diesel mileage isn't really fair because there's about 12% more energy per gallon. Fuel costs per mile is fair enough, and BTU per mile is fair too, but straight MPG with different fuels is silly. You couldn't compare things like CNG, and ethanol or LPG would be tragic. The lack of a throttle, high compression ratio, and the way those combine with a turbo do make diesels really efficient motors, just 12% less efficient than you think.

I think the EPA factors this in because they test the carbon output through the exhaust, so when people buy diesels, they usually end up getting better than the EPA mpg rating. The most efficient gasoline engines are slightly less efficient as the most efficient diesel engines, bsfc of ~235g/kwh (gas) vs ~190g/kwh (diesel), however, the diesel engine is more or less always this efficient, while the gasoline engine is almost always not this efficient.

[tomauto]

Comparing s2000 gearing vs. corvette isn't fair at all. Corvette's 6sp is MADE for cruising on highway...The s2000 gearing is more related to a Indy car 6-7th gear change...super close. The s2000 is much more high strung than a corvette.........Those GM bastards...

[JanGeo]

A high torque engine running at a lower rpm to produce X hp will have less rotational losses than a low torque engine running at a high rpm to produce the same X hp. The higher rpm engine will require more gear reduction to maintain the same speed as the low torque engine which will result in the same torque at the wheel to make the cargo.

[Sludgy]

Originally Posted by Matt Timion

At least that's what I just read on the internet.

Torque helps gas mileage. Horsepower does not. This actually makes sense considering diesel engines get much better gas mileage than gasoline engines.

Now, can anyone give me the physics telling WHY torque increases gas mileage and horsepower does not?

Are there any tricks to increase torque?

I'm not sure about Japanese engines, but big american V8 engines can easily be altered to produce low RPM torque via a cam swap.

Racing engines need lots of valve lift and valve overlap for high speed operation. But big valve lift stresses the valvetrain and the added friction makes it waste more power. Valve overlap can allow part of the intake charge to pass through the cylinder (at low, FE rpms) without actually burning in the cylinder. But valve overlap makes the engine breathe much more ealisy and make a lot of power.

A cam swap with mild lift and no valve overlap can produce FE gains. V-8 cam swaps are cheap, too.

High compression pistons are another way to get better FE. The higher expansion ratio extracts more power during the power stroke. But high compression often causes pinging. New pistons are an expensive change, much more expensive than a cam.

If I were going to trick out a V8 for FE with an unlimited budget, I'd buy the highest compression pistons I could find, and find a custom cam grinding shop to make an Atkinson cycle cam. This would prevent pinging and still have the high compresion ratio.

[thisisntjared]

wow this thread has a lot of fuzzy logic.... i think im just going to sit this one out.