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Old 02-15-2012, 06:31 PM   #1 (permalink)
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Hybrid gasoline-gasoline idea.

This was a reply to the ford I3 1 litre thread i started, but the idea then merged onto a different path.

Originally Posted by Old Tele man View Post
...typically, only 15-20 hp is needed for steady 55-60 mph highway cruising.

...the "extra" hp is required to get out your own way on on-ramps, hills, and passing lethargic snails...ie: when accelerating!
How about a "hybrid" gasoline-gasoline engine! You have your first engine designed to get you up to highway cruising speed in a reasonable amount of time as well as having it on tap for instant power passing and climbing. But once you get to highway speed and switch on cruise control, you instantly get electronically disconnected and switched to your highway engine. A 120cc single cylinder tuned to get ridiculous mileage maxed to 20hp.

Why deal with pumping and frictional losses of a 3 or 4 cylinder "two personality" engine getting both power and mpg? Why try to disable cylinders on a V6 like on another thread?If that suzuki I3 1 litre you say can get 80mpg at a steady state 50mph, how much more will a 120cc 1 cylinder get that is designed only to operate at this narrow margin band of minimal horsepower cruising. 120mpg?

You would have to make some sort of hybrid drivetrain, but the overall cost would be much cheaper than electric-batteries. Something like $400-500 for a nice little moped engine.

Last edited by sheepdog 44; 02-15-2012 at 06:59 PM.. Reason: grammer
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Old 02-15-2012, 07:36 PM   #2 (permalink)
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Make it a little turbo diesel engine and you could really go places.
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Old 02-15-2012, 08:00 PM   #3 (permalink)
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Or just make it a hydraulic hybrid and run the regular engine 1/4 of the time but only at peak BSFC.

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Old 02-15-2012, 08:26 PM   #4 (permalink)
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Or make it electric. Oh...

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Old 02-15-2012, 09:08 PM   #5 (permalink)
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As i understand it, electric cars are the least efficient at highway speeds. On hybrids you have a decent amount of electric torque to get you accelerated past low speeds, and a gas engine that is competent enough by itself to bring you through the highway speeds. Neither of which are only tuned to run optimally in the narrow section of highway cruising. Add more electric power to compensate for only using a 120cc gas engine while cruising, and you add alot more cost and weight to the vehicle. Use only a low horsepower 3 cylinder engine and you get good highway mpg, but terrible low end performance and acceleration.

I would'nt object to a Tribrid electric/1.5litre/120cc. The only goal is to max out highway mpg for the least amount of money like any good ecomod should do. For example:

The VLC used a 250cc motorcycle engine at a max 40 hp but in reality only required 3.5hp to cruise at 50mph. Imagine the mileage that would get with a 60cc engine maxing out at 3.5hp. Running only a 3.5hp engine on that would be implausable, thus a hyrbrid gas/gas engine. The cost of a 60cc would be next to nothing, but a hybrid platform would cost $6,000 or more on a VLC (theoretical guestimates for arguments sake). You could see a city mpg improvement of 20% or more for $6K, or a 20% highway improvement for under $300.

Highway has always been the biggest mpg factor for me as my family lives on the east and west coasts.
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Old 02-16-2012, 10:29 AM   #6 (permalink)
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I think it's a great idea, but I'm not aware of any engines that small that are really tuned for high efficiency.
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Old 02-16-2012, 11:02 AM   #7 (permalink)
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There are some small 20hp diesel engines out there for small tractors, trouble is they are very heavy, Hatz also makes a number of small diesel engines that would work, or you could just go with a single diesel engine and get great mileage while saving the weight of having a 2nd engine, after all diesels work best for highway cruising.
Yanmar makes some really nice 20hp and larger diesel engines that people have put in cars, but they also cost $5,000 or more for a new one and it looks like $1,500 or more for a used one.
Your math is a bit fuzzy tho, I don't think you will get 120mpg out of a "normal" car at highway speeds no matter what the engine you put in there is, figure your current engine is 15-20% efficient and lets say you are getting 30mpg right now (you don't have a vehicle listed so I don't really know what you are getting) but even at 15% efficient 100% would get you 200mpg and that would be -0- losses, cold exhaust, no radiator and a engine that never gets warm, most efficient diesel engine is a huge one on a freight ship and it tops out at 50%, so even if you could scale it down to fit in your car you would still be getting 100mpg, but that 50% is on a large scale with cylinders that are 3 feet across, so 30% from a well tuned diesel is maybe possible? 60mpg at highway speeds... for that getting an old Diesel VW Rabbit might be your best bet and just keep it simple.
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Old 02-16-2012, 11:25 AM   #8 (permalink)
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Replacing your 2.0L with a 1.0L isn't going to net you a 50% increase in FE. Engines are only so efficient. The benefit from using a smaller engine comes from the fact that you're utilizing that engine closer to peak bsfc a higher percentage of the time its in use.
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Old 02-16-2012, 11:27 AM   #9 (permalink)
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It's a good idea, but engines don't just get more and more efficient when they get smaller, there is an optimum size for each power level, 600cc would be about right for a normal car at highway cruise.

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Old 02-16-2012, 01:35 PM   #10 (permalink)
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I haven't been doing any math, so help me figure out if the effort would be worth the possible gains in mpg and savings of this theory in a scientific way. What are the highway mpg improvements to be had by using a small displacement engine: less pumping and frictional losses, price, and better tuning for its purpose.

I want to find the hp required to cruise at 50mph and 65mph for a 95 civic dx/cv/vx. And cross reference it to the fuel consumption at that hp for the donor engine. It has to be cheap, and it doesn't matter where it comes from such as motorcycle, moped, light industrial etc as long as its fuel efficient. The optimal engine would be the Loremo 20hp turbo diesel but i couldn't find the specs for that engine.

Here are just the first examples i could find but I can't translate the fuel consumption of these engines from g/hph or litres a kilowatt hour.




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