High-MPG Corvette

[theunchosen]

Christ is right about the O2 being a problem. Its the reason that electrolysis machines are enormously expensive. The diodes have to be made out of exotic metals that resist corrosion(like your cat but in much greater quantities). An electrolysis device just has a variable voltage output between two diodes. Lab systems will cost a few grand upwards.

[roflwaffle]

Originally Posted by Big Dave

A LS2 makes 405 HP. How many minutes of full power do you suppose that a LS2 actually see in its service life You can get some serious tickets by driving 175 MPH.

Generally speaking, most vettes that aren't bought for the "image" get raced at tracks and such. Heck, someone could gut a Prius and turn it into a race car, but it'd be a bit odd and more expensive than just getting something designed to race, just like swapping a diesel into a vette is a very expensive way to get a ho-hum economy car.

Originally Posted by Big Dave

The diesel is what pushes a 'Vette over 40 MPG. You could put an itty-bitty gas engine in it but I don't think 40 MPG is possible without competition-grade hypermiling. The diesel, with its higher efficiency does it easily.

Proper gearing compared to BSFC is what gets a car like a vette over 40mpg (energy adjusted), regardless of engine type. A chebby gen iv engine at ~250g/kWh will hit ~40mpg@65mph, just like a 4BT at 225g/kWh will hit ~45mpg, and they'll both be at a shade over 30% BTE (diesel has more energy per gallon than gasoline).

Originally Posted by Big Dave

The propane/nitrous is there for bursts of acceleration. I specify nitrous to avoid having to put a bigger turbo on the engine (and accept more turbo lag) for maybe fifteen minutes of raw power a year. Its simply "turbo logic." With a turbo, a small engine can have a "Jeckyll and Hyde" personality. Economical most of the time, but powerful when needed. How many turbo cars are at full boost much of the time? They'd grenade themselves.

If someone only needs 15 minutes of raw power every year then why even both w/ a vette and a 4BT/LSx/whatever? Just go w/o those 15 minutes of power and grab a beater IDI VW. 40+mpg@65mph and it'll only cost a few hundred bucks.

Originally Posted by Big Dave

The little Cummins is straight mechanical and I don't have to mess with electronic controls.

I never understood the preoccupation w/ mechanical versus electrical. It's all basic engineering/physics and it's all gud. All things being equal, electric engine/emissions control is just as reliable as mechanical, although I imagine that it could get pretty hairy given some of the QC seen w/ domestic manufacturers in the past, but that's QC, not some intrinsic mechanical versus electrical problem.

[roflwaffle]

Originally Posted by Frank Lee

I just know how tough it is to hit 40.

That depends on what kind of gearing selection ya have available. With proper gearing, ~40mpg@55-60mph is possible for most modern engines/cars, but w/o it, forget about it.

[theunchosen]

Unless you want the vette for the looks and to be able to get the FE. . .but then. . .its definitely cheaper to just straight up build a custom and then you get more power and more FE for alot less(about the price of your engine it sounds like)

[Frank Lee]

Originally Posted by roflwaffle

That depends on what kind of gearing selection ya have available. With proper gearing, ~40mpg@55-60mph is possible for most modern engines/cars, but w/o it, forget about it.

So the concept of an ideal piston speed for economy is bunk?

[roflwaffle]

Piston speed is o.k. as a rule of thumb, provided the ratio of road load power requirements to displacement stays the same but when we're looking at small'ish vehicles with big engines, or vice versa, it isn't very accurate. If we say a small car with a two liter engine that needs 25hp@55mph has an ideal piston speed of whatever in terms of economy, then if we have a four liter engine in a truck that needs 50hp@55mph the ideal piston speed for economy will be more or less the same.

Otoh, and six liter engine in a vehicle that's only about five hundred pounds more with the same or lower CdA than that of the small car would need to cut engine speed proportionally to the difference in displacement and increase (or decrease it if it needs less power) it proportionally to the extra power required, so it would be moving maybe 2.5 times slower to see optimal efficiency.

There's a problem however if the drop in speed required puts a decent load(torque) level at or less than idle. The solution is to shut off half of the cylinders and all of a sudden a 6L becomes a 3L, with the ~50% increase in mileage mentioned earlier. A large vehicle at 55mph can see a ~40-50% increase in mileage, and something like a vette should be able to see better w/ the right gearing. Something else indicative of poor gearing in terms of efficiency is if mileage doesn't change much w/ speed. ~27-28mpg@75mph is common w/ a vette, so if ~32mpg@55mph is the best that can be done, then we know engine efficiency increased a whole lot because if it was optimal at 55mph it would've been cut in half at 75mpg instead of dropping by ~12%.

[jamesqf]

Originally Posted by theunchosen

Christ is right about the O2 being a problem. Its the reason that electrolysis machines are enormously expensive. The diodes have to be made out of exotic metals that resist corrosion(like your cat but in much greater quantities).

Yet welding oxygen is stored in non-exotic metal cylinders, and is not all that expensive - something like $25 to refill a cylinder, IIRC.

[Christ]

Originally Posted by jamesqf

Yet welding oxygen is stored in non-exotic metal cylinders, and is not all that expensive - something like $25 to refill a cylinder, IIRC.

Yeah, it's about $25... and welding grade O2 isn't pure oxygen, either. I don't remember if they actually add anything to it, but it's not even close (relative to terminology) to medical grade (pure, for all intents and purposes) oxygen supplies, which are stored in plain old aluminum tanks that have been vacuumed and boiled (heated to just over 100*C) prior to filling.

The problem doesn't rest with O2 being reactive to metals as a whole, it comes with O2 reacting pre-combustion with other elements in the combustion chamber, such as nitrogen and what not in the air. Pure oxygen is in fact an oxidizer, but the maintained reason for the nitrous in nitrous oxide is stability of the gas itself.

[MechEngVT]

Originally Posted by Christ

Yeah, it's about $25... and welding grade O2 isn't pure oxygen, either. I don't remember if they actually add anything to it, but it's not even close (relative to terminology) to medical grade (pure, for all intents and purposes) oxygen supplies, which are stored in plain old aluminum tanks that have been vacuumed and boiled (heated to just over 100*C) prior to filling.

The problem doesn't rest with O2 being reactive to metals as a whole, it comes with O2 reacting pre-combustion with other elements in the combustion chamber, such as nitrogen and what not in the air. Pure oxygen is in fact an oxidizer, but the maintained reason for the nitrous in nitrous oxide is stability of the gas itself.

Elemental oxygen is highly reactive. So much so that at standard temperature and pressure it forms a diatomic bond with itself to form the stable O2. At STP oxidizing by the presence of 20% O2 is a slow process for most materials.

The elevated temperatures will cause the O2 to split into 2x O- which is a highly reactive oxidizing agent. It's been a while so I don't recall how easily O2 separates when compared to N2O.

I've read somewhere that oxygen for industrial purposes (welding, smelting) has more critical purity requirements than medical oxygen. I've also read that there's no difference between industrial/medical oxygen except medical applications include secondary humidification after packaging the O2 from a common source.

I think roflwaffle is right; maintaining an "ideal" piston speed is only relevant if that speed is maintained at a high load. Almost all roadworthy vehicles are a degree overpowered and maintaining that load would result in acceleration, not steady-state operation. Over-gearing to run at a sub-optimal engine speed would be more efficient for throttled engines, not sure how it would impact diesels.

[Who]

A GM 3800 V6 in the right tune with really long gearing...