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Old 05-07-2022, 04:55 PM   #1 (permalink)
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Turbocharging with no intercooler, petrol/ethanol

So far this is just theoretical, I don't think that I'm going to have budget, time or place to do anything concrete any time in near future, but I'd like to know if I'm after something. There is so much technical knowledge here, that someone got to know or have a relevant argument's at least.

This hypohesis is mainly for ethanol fuel, I think. As we know, ethanol's energy content is lower than petrol. But, because of that it burns a bit cooler. It have a higher octane number too, so it's possible to use higher compression ratios.

So, if a engine running with ethanol is turbocharged with relatively low pressure, let say 7-8psi/0.5bar and no cooling is used, we have a warmer intake air, better fill in cylinder and part of the wasted energy in exhaust gasses is used to produce power, is it going to be better average mpg?

Most of the small modern engines have turbo, but that's maybe mainly to have efficiency benefits of small engine but the power of a bit bigger engine, to make customers happy? But is it possible to charge an engine only to use lesser fuel? Or has it got to be done with longer gear ratios to have any benefits?


Last edited by J4nn3R; 05-07-2022 at 04:58 PM.. Reason: Edit: question about gear ratios added
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Old 05-07-2022, 08:06 PM   #2 (permalink)
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The first turbocharged cars from GM used ethanol to counter detonation.

https://www.oldcarsweekly.com/features/olds-f85-jetfire
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The little Jetfire was based on Oldsmobile’s F-85 series. As a result, it used the same aluminum-block 215-cid V-8 that survives to this day in the British Land Rovers and Range Rovers (albeit bored out a bit).

Oldsmobile added a waste gate, kept this engine’s 10.25:1 compression ratio and suppressed detonation with a water-and-alcohol injection system. This system included a tank under the hood for “Turbo-Rocket” fluid. The turbocharger’s boost was limited to 6 psi.
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Old 05-08-2022, 01:57 AM   #3 (permalink)
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I don't think so. Adding air (boost) also increases the amount of fuel. Doing little to change the efficiency of the reaction. Not all cars have responded positively to a warm air intake either. That could be partly dependent on the filter location itself though.

I would be curious to know if running the engine at 0 psi of boost would be beneficial.
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Old 05-08-2022, 02:10 AM   #4 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
I would be curious to know if running the engine at 0 psi of boost would be beneficial.
You mean like to decrease pumping losses? With my very limited knowledge about that, I would think that this kind of slight improvement on intake side could be canceled because of a bit higher resistance on exhaust side. Maybe?

But as you say, boost equals more oxygen equals more fuel to burn, so it could be the situation that any modification that increases power needs longer gear ratios to make any good for fuel economy. With boosted air you don't have to open throttle that much to maintain certain speed, but the amount of air flowing to chambers is same, so the amount of fuel is aproximately same. Or could be?
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Old 05-08-2022, 11:33 AM   #5 (permalink)
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It'll be more efficient at high load if you can radically modify the cam profiles or have variable valve duration. A turbocharged "Atkinson" cycle engine is essentially a turbocompound engine (turbocharger takes over compression duties, cylinder is used more for expansion than compression).

The problem is with most engines, ANY boost even on high duration race cams is going to be more torque than you need to cruise.

You need VVEL/Valvematic/Valvetronic/Multiair to be able to cut volumetric efficiency enough, but at low load the exhaust has less waste energy and the engine's friction is the greatest source of loss so the improvement will be very minimal.

Last edited by serialk11r; 05-08-2022 at 12:08 PM..
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Old 05-08-2022, 12:35 PM   #6 (permalink)
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Great question and I've tested this on my Talon.

Ethanol takes about 40-50% more fuel for the same power. So, you will use more of it then petrol at the same A/F ratio equivalent.

Even increasing the engines compression will give you a very limited increase in efficiency.

The only way to increase FE at 7psi is to run a lean-burn engine.

On my setup I can run 7psi at 20:1 A/F ratio on petrol.

At 7psi with a 20:1 A/F at a BSFC of .45 lb/hr vs 7psi with a 12.5 A/F ratio at a BSFC of .55 the gain in torque is around 12 ft/lbs. The amount of fuel saved is 18 lbs/hr. around 3 gallons with a gain in torque of 12 ft/lbs. This is a petrol-to-petrol comparison at 3500rpm loaded on a 2.0L. engine with a total 100 ft/lbs of torque at the BSFC of .45 lbs/hr.

So, with the above example you could lower the boost to say 6psi and still have the same total amount of torque and increase the FE.

With a lean burn engine at this low rpm you have to increase tumble and swirl due to the low velocity of the air and a bunch of other modifications to make this work.

Forgot to add this as you can see its 100 ft/lbs of torque total so this would be used only to climb a hill or getting up to speed on a on ramp etc.
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Old 05-08-2022, 01:25 PM   #7 (permalink)
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Great answer from pgfpro.

Most cars lose a lot of efficiency under boost because of two things:
1) They need to retard ignition timing.
2) They need to run rich.

They also lose efficiency overall because 3) they need to have lower static compression ratios.

All three items are to fight detonation. Additionally, higher intake air temperatures also make detonation more likely, resulting in the need to pull more timing and add more fuel.

Running a sufficiently high octane fuel would fix this. Lean burn additionally reduces combustion temperature.

Get detonation under control, so you can avoid running rich, and keep spark timing optimal, and you should see decent efficiency gains both from harvesting exhaust energy with the turbo, but also from increased cylinder pressure and effective compression. Low static compression ratios can be worked around by running Miller cycle (effectively Atkinson cycle + forced induction).
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Old 05-08-2022, 02:20 PM   #8 (permalink)
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Sounds like 1980s GM cars that were turbo charged, but they didn't use ethanol.
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Old 05-08-2022, 02:39 PM   #9 (permalink)
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Quote:
Originally Posted by pgfpro View Post
The only way to increase FE at 7psi is to run a lean-burn engine.
Lot of great information, but that's the thing, I guess? Engine should have certain qualities and needs serious remapping. But, all in all, with ethanol fuel most efficient engine could be a boosted one. Even if it have to be build from scratch with that in mind, one will not make any more miles pet gallon just bolting a Garret on and filling tank with E85. Anyhow, interesting. In big picture ethanol is so marginal, that I think that there is no pure ethanol engines from major companies.
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Old 05-08-2022, 04:36 PM   #10 (permalink)
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Quote:
Originally Posted by J4nn3R View Post
In big picture ethanol is so marginal, that I think that there is no pure ethanol engines from major companies.

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