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Discussion Starter · #1 ·
The latest advertising brochure from Toyota on the 2006 Prius states that the compression ratio of its little gasoline engine is 13.0. Needless to say, I was astounded to read this and am almost certain that it is a typographical error for the correct value of, say, 10.3.

Does anyone know what the real compression ratio is in this engine?

I believe that you can run a Diesel engine on a compression ratio of 17 or so.

Thanks,
Jim (who is almost ready to buy a Prius)
 

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Engine is an atkinson/miller, not an Otto. The Prius engine extends the power stroke compared to the compression stroke by effectively shortening the compression stroke. What it does is keep the intake valve open partly into the compression stroke which causes some of the fuel/air mixture to 'leak' back into the intake maifold. It is true that the pressure at TDC is probably the same as any other regular gasoline burning engine, otherwise the fuel mixture would self detonate, but it's the effective stroke ratios that are being compared.

This provides increased engine efficiency, but reduces power output. The reduced power output is compensated using electric motors.

See Grahm's original site for details: http://prius.ecrostech.com/original/PriusFrames.htm
 

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The engine in the Prius is not adaptable to any other car. It was designed to run in conjunction with the electrics. Does not even have the standard 12 volt starter that the Honda Civic hybrid is supplied with. The Honda uses both methods of starting and stopping the engine but the civic's was adapted from the standard engine of the "normal" car.
 

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Discussion Starter · #5 ·
DanMan32 said:
Engine is an atkinson/miller, not an Otto. The Prius engine extends the power stroke compared to the compression stroke by effectively shortening the compression stroke. What it does is keep the intake valve open partly into the compression stroke which causes some of the fuel/air mixture to 'leak' back into the intake maifold. It is true that the pressure at TDC is probably the same as any other regular gasoline burning engine, otherwise the fuel mixture would self detonate, but it's the effective stroke ratios that are being compared.

This provides increased engine efficiency, but reduces power output. The reduced power output is compensated using electric motors.

See Grahm's original site for details: http://prius.ecrostech.com/original/PriusFrames.htm

Lot of typing, lot of words, but no answer to my question, which was: "What is the compression ratio?????"
 

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jamesfchumbley said:
The latest advertising brochure from Toyota on the 2006 Prius states that the compression ratio of its little gasoline engine is 13.0. Needless to say, I was astounded to read this and am almost certain that it is a typographical error for the correct value of, say, 10.3.

Does anyone know what the real compression ratio is in this engine?

I believe that you can run a Diesel engine on a compression ratio of 17 or so.

Thanks,
Jim (who is almost ready to buy a Prius)
You answered your own question: 13 I underlined the sentence and bolded 13 in the quote so you could verify this. What I got from your question since you stated that it was said to be 13 was to confirm it was 13 and why, which is what I answered.
 

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Does it not have different compression and expansion ratios, which is the whole point of the atkinson cycle? It would make more sense if the compression ratio were about 10 with an expansion ratio of 13.
 

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hyperion said:
And if it was a standard gas engine at 13/1 it would require the highest octane gas available maybe even 100 octane AV Gas.
Hate to be contrary, but that is not always true.

I live at 1700ft, but I mostly drive near sea level. Even in my old TransAM (12:1)and Porsches (13:1), I could usually get away with burning 87octane. When I went to the high desert like Willow Springs, I would switch to 91. Remember, the reason we use higher octanes is anti-knock and anti-ping. True, there may have been a reduction in absolute horsepower, but in typical SoCal slog-traffic, who notices a loss of 5HP from 147?

Walt
 

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Discussion Starter · #11 ·
richard schumacher said:
Does it not have different compression and expansion ratios, which is the whole point of the atkinson cycle? It would make more sense if the compression ratio were about 10 with an expansion ratio of 13.
Here is a URL for a very beautiful moving diagram (full motion!) of a true Atkinson cycle engine, showing the split connecting rod and the method of achieving four piston strokes with ONE(!) rotation of the crankshaft. Well worth looking at:

http://www.keveney.com/Atkinson.html

Can someone explain in some detail for me how the engine in the Prius works? From the discussion I've seen here, it seems to be more of a Miller cycle than an Atkinson, but I don't really understand the Prius engine yet.

Also, could some mechanical gal or guy sketch the different strokes and the thermodynamic cycle for me? I can't find it anywhere on the internet.

Thanks a lot,
Jim
 

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Discussion Starter · #12 ·
jamesfchumbley said:
The latest advertising brochure from Toyota on the 2006 Prius states that the compression ratio of its little gasoline engine is 13.0. Needless to say, I was astounded to read this and am almost certain that it is a typographical error for the correct value of, say, 10.3.

Does anyone know what the real compression ratio is in this engine?

I believe that you can run a Diesel engine on a compression ratio of 17 or so.

Thanks,
Jim (who is almost ready to buy a Prius)

I just ran across the Car & Driver review of the 2004 model Prius at: http://www.caranddriver.com/roadtests/7 ... prius.html . In it they list the following technical specifications:

ENGINE
Type inline-4, aluminum block and head
Bore x stroke 2.95 x 3.33, 75.0 x 84.7mm
Displacement 91 cu in, 1497cc
Expansion ratio 13.0:1
Effective Atkinson-cycle compression ratio 9.5:1
Fuel-delivery system port injection
Valve gear chain-driven double overhead cams, 4 valves per cylinder, hydraulic lifters, variable intake-valve timing
Power (SAE net) 76 bhp @ 5000 rpm
Torque (SAE net) 82 lb-ft @ 4200 rpm

They quote the compression ratio as 9.5 to 1, which seems like a reasonable number to me. Any comments on why this number would be so different from the Toyota-supplied number for the 2006 model's compression ratio?
 

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Compression ratio

Expansion ratio is 13.0. Compression ratio is "about" 9.5. Since the Prius has "Variable Valve Timing with Intelligence" (VVT-i), the true compression ratio changes based on operating conditions.

Compression ratio is usually calculated as volume change from bottom of the piston stroke divided by volume at the top of the piston stroke. This formula gives 13.0. But if the valve closes late on the upstroke, it effectively reduces the true amount of mixture compression.

87 octance and 13.0 compression ratio are incompatible.
 

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That's what I was saying. The pressure at top dead center in PSI or Pascals is normal for a gasoline engine, I see that they rate it at a bit less. If it weren't, you'd have to use premium fuel, otherwise the fuel/air mixture would ignight before the spark plugs told it to.

What I believe would make the system even more efficient, or at least cleaner is if the fuel injection occured directly into the cylinder only after the intake valve closed. As it is now, the fuel is injected in the manifold just before the intake valve so the fuel has to be injected while the intake valve is open. This means that some fueled air leaks back out of the valve during the early part of the compression stroke while the intake valve is open. If the fuel could be injected in the cylinder once the valve is closed, no unburned fuel would be found in the intake manifold, except for what is sucked in from the fueltank evaporative emissions system.
 

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Or even better, Grahm's site, though some of the links do not work anymore:

http://prius.ecrostech.com/original/PriusFrames.htm
http://prius.ecrostech.com/original/Und ... ustion.htm

Also the link Ken gave shows the dynamics in graphs. It is important to read the text thoroughly.

I think the problem is that Toyota used the term Compression ratio for their specs, when really they are giving the spec for the expansion ratio. I believe they can get away with that because the compression stroke piston travel is the same as the expansion (power stroke) piston travel, but the actual compression travel is reduced by keeping the intake valve open. The piston travel during actual compression is shorter.
 

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I believe that you can run a Diesel engine on a compression ratio of 17 or so.
More like 19.5:1 for a diesel :)

Prius isn't in such demand in the UK because of good diesels... My previous car got pretty similar mpg, admittedly it was a smaller car. Interesting to note though that burning a litre of petrol produces about a third more C02 than a litre of petrol I think.
 

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Danman - it's denser, and as more molecules with more carbon atoms attached afaik.

My Seat Ibiza 1.9TDI had an official combined fuel economy figure of 58mpg, and my Prius 61. The CO2 consumption figures however say 134g/km for the Seat and 104g/km for the Prius.

Toyota make a city car called the Aygo in petrol and diesel versions. The petrol gets something like 61mpg and the diesel 69 - but they are both rated at 109g/km. So in that case, there's less difference. Either the Prius or Toyota's diesels are very good at minimising CO2.

Perhaps this warrants a new thread, if you guys are interested in talking about it. My American father-in-law is quite obsessed with mileage, emissions and in particualr diesels, he talked about cars most of the time he was over here :)
 
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