Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Gas-Electric propulsion system for 3.5 ton conversion van

McDesign · 04-14-2016, 06:34 PM

Paul - I'm a mechanical engineer, so you can trust me. I have like, patents and stuff.
Long post to respond to your thorough post.

Think about and analyze what you are trying to do from an energy balance standpoint - thinking and calculating is free!

First - locomotives are diesel-electric simply to eliminate the extremely torque mechanical connection between a low-speed 3000 hp and the multiple drive axles - far more dependable to have an electric motor in each axle. THERE IS NO EFFICIENCY "GAIN" IN THIS - it's done for reliability and cost - locomotive fuel was a few cents per gallon when they were designed.

Back to you.

You need to move x weight at y speed. You also have to accelerate that weight. You have to drag that weight at constant speed up hills. You have to push it through the air. You have to overcome drivetrain losses (a surprising amount with a non-lockup converter and in the rear-axle gearing). Hysteresis losses in your tires - all that.

OK. So after that, you have an energy budget. Now - what can supply that energy?

Internal combustion engines (gas or diesel) are easy to compare in terms of lbs of fuel per horsepower per hour - that's BSFC, or "brake specific fuel consumption". Broadly speaking, that's a number between .4 and .5, and is ~ the inverse of the torque curve of the engine - max efficiency happens near peak torque for a given engine.

Since you've calculated your energy budget, you can figure what's the BEST possible gas or diesel consumption for your needs over an average driving cycle by a theoretical engine - I'll bet you'll find for your situation, with that engine displacement and old-tech cylinder head design and a carb, it's around 12-15 mpg for gas, maybe ~3-5 MPG more for an injected diesel.

OK - so that's the best a well-tuned "theoretical" engine can do. You want to replace that with some other source. Let's say you could have a ~20-30 horsepower common-rail injected diesel generator running at perfect, consistent loading, such that you got maybe ~.35 or even .3 BSFC. Great - now you're making power (electric, not mechanical) at higher efficiency from your fuel.

But - you have to store that electricity in expensive or heavy batteries, so you can use a hundred horsepower or so when you need it to accelerate or climb a hill. Also - you have to convert that electricity into a form (voltage and frequency) that can be used by your motors, and then the motors have to generate mechanical torque from electrical energy. That will cost you 20-40%, which wipes out your great BSFC number back to where it was with a well-tuned IC engine.

All that work, and no better.

Wood gas, you'll find, is EXTREMELY limited as to power and efficiency -there are several folks who have build running versions, and this has been their experience. Also, all the anecdotal evidence form German and eastern Europe during WW2.

Full electric? I went through the exercise completely about four years go, as a rigorous engineering project. I wanted to convert my '91 Accord coupe to electric with a 105 mile range.

Lithium iron phosphate batteries of the appropriate size were $26,000. Absolutely no joke. Just the batteries - not the ~$4-8,000 for the rest of the conversion.

I realized i could never drive enough to save enough on gas to make back the cost of "pre-paying" for my energy.

Instead, I went through an annotated program of step-by-step aerodynamic improvements - I think there is a thread in here. I went from 27 to 36 mpg in my particular driving mix, and can get ~44 mpg at 75 mph and approaching 50 mpg at 55 mph on I-95 between South Carolina and Maryland - but that's dangerously slow.

So - there is no easy solution to what you want to do, even if you had a $30-50K budget.

But - you must do realistic calculation and prove your plan to yourself before you spend a dime - wishful thinking will cost you money!

04-14-2016, 06:34 PM	#53 (permalink)
McDesign EcoModding Lurker Join Date: Oct 2012 Location: Atlanta, GA Posts: 32 Thanks: 0 Thanked 16 Times in 9 Posts	Paul - I'm a mechanical engineer, so you can trust me. I have like, patents and stuff. Long post to respond to your thorough post. Think about and analyze what you are trying to do from an energy balance standpoint - thinking and calculating is free! First - locomotives are diesel-electric simply to eliminate the extremely torque mechanical connection between a low-speed 3000 hp and the multiple drive axles - far more dependable to have an electric motor in each axle. THERE IS NO EFFICIENCY "GAIN" IN THIS - it's done for reliability and cost - locomotive fuel was a few cents per gallon when they were designed. Back to you. You need to move x weight at y speed. You also have to accelerate that weight. You have to drag that weight at constant speed up hills. You have to push it through the air. You have to overcome drivetrain losses (a surprising amount with a non-lockup converter and in the rear-axle gearing). Hysteresis losses in your tires - all that. OK. So after that, you have an energy budget. Now - what can supply that energy? Internal combustion engines (gas or diesel) are easy to compare in terms of lbs of fuel per horsepower per hour - that's BSFC, or "brake specific fuel consumption". Broadly speaking, that's a number between .4 and .5, and is ~ the inverse of the torque curve of the engine - max efficiency happens near peak torque for a given engine. Since you've calculated your energy budget, you can figure what's the BEST possible gas or diesel consumption for your needs over an average driving cycle by a theoretical engine - I'll bet you'll find for your situation, with that engine displacement and old-tech cylinder head design and a carb, it's around 12-15 mpg for gas, maybe ~3-5 MPG more for an injected diesel. OK - so that's the best a well-tuned "theoretical" engine can do. You want to replace that with some other source. Let's say you could have a ~20-30 horsepower common-rail injected diesel generator running at perfect, consistent loading, such that you got maybe ~.35 or even .3 BSFC. Great - now you're making power (electric, not mechanical) at higher efficiency from your fuel. But - you have to store that electricity in expensive or heavy batteries, so you can use a hundred horsepower or so when you need it to accelerate or climb a hill. Also - you have to convert that electricity into a form (voltage and frequency) that can be used by your motors, and then the motors have to generate mechanical torque from electrical energy. That will cost you 20-40%, which wipes out your great BSFC number back to where it was with a well-tuned IC engine. All that work, and no better. Wood gas, you'll find, is EXTREMELY limited as to power and efficiency -there are several folks who have build running versions, and this has been their experience. Also, all the anecdotal evidence form German and eastern Europe during WW2. Full electric? I went through the exercise completely about four years go, as a rigorous engineering project. I wanted to convert my '91 Accord coupe to electric with a 105 mile range. Lithium iron phosphate batteries of the appropriate size were $26,000. Absolutely no joke. Just the batteries - not the ~$4-8,000 for the rest of the conversion. I realized i could never drive enough to save enough on gas to make back the cost of "pre-paying" for my energy. Instead, I went through an annotated program of step-by-step aerodynamic improvements - I think there is a thread in here. I went from 27 to 36 mpg in my particular driving mix, and can get ~44 mpg at 75 mph and approaching 50 mpg at 55 mph on I-95 between South Carolina and Maryland - but that's dangerously slow. So - there is no easy solution to what you want to do, even if you had a $30-50K budget. But - you must do realistic calculation and prove your plan to yourself before you spend a dime - wishful thinking will cost you money!