Article: adding a turbo to improve efficiency

[tjts1]

Originally Posted by diesel_john

"Unfortunately, for us, auto mfrs like to use fuel for it's heat capacitance rather than it's combustible characteristics "

could you explain, i don't understand what heat capacitance is.
(the learning gradient is too steep for me)

In order to prevent detonation under boost in a turbocharged engine during high load, the computer will sometimes increase the air fuel ratio from the stoichiometric 14.7 to 1 ratio to something closer to 13 or 12 to one. Because liquid fuel has higher latent heat than air, the extra fuel injected into the engine is used to absorb heat from the combustion chamber and prevent detonation. This fuel does not get burned in the combustion chamber (not enough oxygen) so its wasted out the exhaust in the form of extra pollution. One solution to this problem is to maintain the 14.7 AFR even at high load and inject water to prevent detonation instead of fuel.
This is was also common practice in most non turbo fuel injected engine but to a lesser extent until very recently. Manufacturers got away with this practice because EPA emissions test did not include a full load (throttle to the floor) cycle.

[trebuchet03]

Originally Posted by diesel_john

"Unfortunately, for us, auto mfrs like to use fuel for it's heat capacitance rather than it's combustible characteristics "

could you explain, i don't understand what heat capacitance is.
(the learning gradient is too steep for me)

tj got it.....

To add to why they do it.... It's a way to increase horsepower by having dynamic compression higher than it should be ("should" meaning low enough such that extra fuel isn't needed to remove heat)

[Daox]

So, I was reading a little bit about BSFC (brake specific fuel consumption, or the amount of fuel used per horsepower) and saw that the lowest BSFC occurs at peak torque (assuming the air fuel ratio remains constant). With this in mind, we could build an engine with a turbocharger that runs at peak torque while remaining at 14.7:1 by altering boost pressure.

In the performance world, they set the boost pressure as a constant for performance reasons. This requires the air fuel ratio to be modified to prevent detonation as mentioned above. However, if we were simply to lower the boost pressure to avoid detonation, we could maintain the highest possible torque the engine is capable of @ 14.7:1 air fuel ratio. This would therefore give us the lowest possible BSFC. Now, keep in mind that this would only be at WOT (no different from any other engine), so pulse and glide would need to be used. This is also where gearing becomes so important, especially for highway driving so you don't have to P&G everywhere.

This also brings up a few other things. For example, what if your engine can handle a lot more torque? Now, you have a beast of an engine in your car and the added torque now creates wheel spin because your accelerating at (near) WOT. Well, now we're back to using a smaller engine. Also, what if your engine can't handle more torque? Well, then you won't see much in the way of gains. You could therefore look into alternatives like head modifications to supress detonation, coating pistons and chambers with ceramic coatings, etc.

[zjrog]

Legendary mechanic Smokey Yunick developed a low pressure turbo engine 30+ years ago. Scoffed at by the mainstream because he was using a turbo, but not as much for what most folks used one for. His purpose was to heat the air more than the higher pressure to make horsepower.

Anyway, years ago I wanted to turbo my old minivan. THe V6 was OK, and I had modded it as much as I could short of tearing it down. But I wanted more lowend power. I decided a smaller turbo was going to be acceptable, as I was only going to go with 6-7 psi. What I would require was slightly larger fuel injectors, which actually worked quite well without the turbo (once I pulled my head out and settled a fuel pressure problem...).

Most performance guys factor in extra fuel for the purpose of cooling the fuel charge. Wasteful, yes. Not as effective as nitrous (not in large quantities at least). But useful as a form of "chemical intercooling".

Hmmm, I still have the intercooler I was going to use, out in the garage.

[boxchain]

Originally Posted by Daox

So, I was reading a little bit about BSFC (brake specific fuel consumption, or the amount of fuel used per horsepower) and saw that the lowest BSFC occurs at peak torque (assuming the air fuel ratio remains constant).

Thank you. I've been telling people this for years but never found any way to explain it other than through the theories of physics, thermodynamics, and common sense. Also, I'd love to see numbers (per usual )

This bolsters my theory that a LPT that shifts the peak torque down in terms of engine speed will greatly help FE on acceleration.

[diesel_john]

one of the first turbo'd US production engines i remember was about a '62 buick starfire. it had water injection. are telling me that the first one was done right, and we have gone down hill ever since.


On a different note: why not take two pistons, rods and 4 cam followers out of one these tubo'd 4cylinder's. Wouldn't that keep it out of the waste gate, and on low boost and be eco as heck?

[trebuchet03]

Originally Posted by zjrog

Legendary mechanic Smokey Yunick developed a low pressure turbo engine 30+ years ago. Scoffed at by the mainstream because he was using a turbo, but not as much for what most folks used one for. His purpose was to heat the air more than the higher pressure to make horsepower.

He didn't call it a turbo - he was adamant about calling it a homogenizer. Fuel was injected BEFORE the turbo I got to speak with someone that worked with him many many years ago. He said the car worked great, got extreme FE and good power but it had one critical problem - it wasn't reliable. He had said the engine was being rebuilt quite frequently.

one of the first turbo'd US production engines i remember was about a '62 buick starfire. it had water injection. telling me that the first one was done right, and we have gone down hill ever since.

I personally wouldn't say requiring water injection means it was done right... WI is a band aid because there's too much heat. Perhaps the same setup on modern fuels with modern engine management on a modern engine would fare differently though

[LostCause]

I think the best technological implementation of a turbo to improve fuel economy would create one hell of a complicated engine.

First, take a small atkinson cycle engine - 13:1 CR with an effective 8:1 CR. Use a small, efficient turbo ducted through an intercooler to raise the effective CR to ~10:1 (i.e. miller cycle). Use variable valve timing to increase the CR to 13:1 w/ turbo (which should already be operating in its most efficient range) and switch the injectors over to feed from a smaller tank holding 91 - 100 (avgas?) octane fuel during acceleration. Route the spent gasses through the primary turbo and feed the residual gas into a turbocompound that sends power back into the flywheel.

Ofcourse, the impact on backpressure will have to be balanced with gains, but it's an orgy for the mind . The coolest implementation I have seen so far is the Napier Nomad engine, even though it's a two stroke. Essentially, a diesel engine is mated to a turbine that drives both a compressor and a CVT attached to the crankshaft. During light cruise, the turbine powers the efficient axial compressor and feeds residual energy back into the flywheel. During takeoff when extra power is needed, fuel is dumped into the turbine making a temporary (and inefficient) turboshaft engine.

Link: http://en.wikipedia.org/wiki/Napier_Nomad

- LostCause

[trebuchet03]

Originally Posted by LostCause

I think the best technological implementation of a turbo to improve fuel economy would create one hell of a complicated engine.

First, take a small atkinson cycle engine - 13:1 CR with an effective 8:1 CR. Use a small, efficient turbo ducted through an intercooler to raise the effective CR to ~10:1 (i.e. miller cycle). Use variable valve timing to increase the CR to 13:1 w/ turbo (which should already be operating in its most efficient range) and switch the injectors over to feed from a smaller tank holding 91 - 100 (avgas?) octane fuel during acceleration. Route the spent gasses through the primary turbo and feed the residual gas into a turbocompound that sends power back into the flywheel.

I really like this idea... But I'd say remove the separate fuel tank and such....

Just go for a miller cycle engine with variable valve timing. It's probably not very difficult to have a valve profile that switches the engine from Atkinson to Otto while off boost... And considering Atkinson cycle isn't new and VVT isn't new and both concepts are implemented into mass produced vehicles - I wouldn't call it complicated at all

Alas, this isn't something that's easily DIYable It sorta has to start from the mfr...

[LostCause]

Thanks. I figured high octane gas would be needed for the super high CR, but water injection may work. I like to think big, not practical . I suppose I might as well aim for a fusion reactor .

I was just thinking, its 1:00am on the west coast here so it must be 4:00am over there. Holy crap man, get some sleep! Forum, so, addicting...

- Lost Cause