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Old 05-13-2011, 05:46 PM   #21 (permalink)
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Originally Posted by bwilson4web View Post
...
ps. I don't know if you are interested but the Prius Atkinson cycle takes the line from 1 to 2 and breaks it into two sections...
That's a nice writeup Bob. Just to clarify there are lots of cars running a "modern" atkinson cycle aside from the prius:
Atkinson cycle - Wikipedia, the free encyclopedia

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Old 05-13-2011, 05:53 PM   #22 (permalink)
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+1 Bob, excellent
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Old 05-13-2011, 05:53 PM   #23 (permalink)
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Originally Posted by dcb View Post
That's a nice writeup Bob. Just to clarify there are lots of cars running a "modern" atkinson cycle aside from the prius:
Atkinson cycle - Wikipedia, the free encyclopedia
If they get the variable intake and duration valve systems worked out, Atlkinson will be the norm, not the exception ... and I can't wait to see it happen.

When I was offered a chance to buy an early 2010 Prius and learned they only have variable angle valves, not variable duration, I almost walked away from the deal. Even now, our 1.5L, 03 Prius and the 1.8L, 10 Prius are getting 52 MPG. Of course the 1.8L has more space and does it 5 mph faster BUT I wanted 60 MPG. <sigh!>

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Old 05-13-2011, 06:00 PM   #24 (permalink)
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Originally Posted by bwilson4web View Post
...I wanted 60 MPG. <sigh!>..
Of course that is where adjusting the nut behind the wheel comes in mighty handy
EcoModder Fleet list - EcoModder.com

I cant wait for better valve operation either.
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Old 05-13-2011, 06:01 PM   #25 (permalink)
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Quote:
Originally Posted by bwilson4web View Post
If they get the variable intake and duration valve systems worked out, Atlkinson will be the norm, not the exception ... and I can't wait to see it happen.

When I was offered a chance to buy an early 2010 Prius and learned they only have variable angle valves, not variable duration, I almost walked away from the deal. Even now, our 1.5L, 03 Prius and the 1.8L, 10 Prius are getting 52 MPG. Of course the 1.8L has more space and does it 5 mph faster BUT I wanted 60 MPG. <sigh!>

Bob Wilson
Are the Prius' sold here different than the overseas units? I notice that they mpg better than our units; or am I looking at Imperial versus US units of measure.
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Old 05-14-2011, 02:43 AM   #26 (permalink)
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Quote:
Originally Posted by cleanspeed1 View Post
Are the Prius' sold here different than the overseas units? I notice that they mpg better than our units; or am I looking at Imperial versus US units of measure.
To the best of my knowledge, the same but it really takes friends overseas to check. For example, the North American NHW11 has a hydrocarbon converter not found overseas. However, they had an optional fold-down rear seat. The North American NHW20 has a thermos for hot coolant that is missing in the EU and Japanese NHW20s. Apparently the ZVW30 can have a 'heads up' display that is not available in North America. But the fundamental vehicles and drive trains are the same.

Now the EU and Japanese mileage testing protocols are different from ours and more different after the EPA 'adjusted' them in 2008. So even after adjusting for gasoline units, the respective government tests are different and ours are more conservative. Funny, the modified EPA tests have not really impacted the actual mileage. <wink>

Go to Fuel Economy and compare the EPA numbers versus user mileage for:
  • 50(EPA) vs 48.8(139 vehicles) - 2010 Prius
  • 41(EPA) vs 49.1(16 vehicles) - 2010 Insight
  • 34(EPA) vs 44.8(13 vehicles) - 2010 Jetta TDI
So the 2008 'adjustment' fixed the Prius numbers but the Honda Insight and Jetta TDI still look bad. What is really sad is what happened to the NHW11 Prius:
  • 48(old EPA) vs 41(new EPA) vs 45.4(24 vehicles) - 2003 Prius
The EPA error has flipped to the other side.

Bob Wilson
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Old 05-14-2011, 11:19 PM   #27 (permalink)
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Found an interesting post in USENET to complement this thread. In particular, the first link
Quote:
Originally Posted by Bruce_Richmond
Funny, I just used some of the same links in a discussion in
alt.global-warming.
Should switch gasoline to 100% ethanol/You switch, retard. - alt.global-warming | Google Groups

(This included a useful engine calculator:
Power Cycle Analysis Online Calculator ...
This allows folks to change different ICE parameters and see the effect. rjw)

I agree the p-v chart is wrong and the normal transition can be seen
in figure 3.8 of your previous link.
http://web.mit.edu/16.unified/www/SP...oIdeal_web.jpg

The horizontal line from 3 to 4 is usually used in the cycle of a
diesel. The rational is that in a gasoline engine the fuel burns
almost instantly giving a vertical line for the rise in pressure.
With the diesel the fuel is injected over a period of time which
mantains a constant pressure during the first part of the power
stroke. Here is a p-v diagram for an Atkinson diesel:
Ern's Blog

Here is an article that gives some info about how the Atkinson cycle
is applied to the Prius.
Hybrid Tech - Honda's Insight and Toyota's Prius - page 2 | Automotive Industries

I also found out that a true Atkinson cycle engine involves more than
just variable valve timing, it actually changes the lengths of the
strokes, thus avoiding the pumping losses of drawing air in and then
pumping it out before closing the intake valve.
Animated Engines, Atkinson

Still the valve timing trick is a lot better than nothing.
Some of the articles are a little dated such as the 1.5L Prius engine analysis. Still, it is a good collection of URLs . . . Thanks to Bruce Richmond.

Bob Wilson
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Old 05-18-2011, 12:18 PM   #28 (permalink)
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Getting back to the direct injection discussion (if you don't mind me rewinding the thread a bit here), the reason that diesels can run lean mixtures more easily is because the intake air charge is compressed to a temperature that is above the autoignition temperature of the fuel.

Since diesels are essentially unthrottled, a full air charge enters the combustion chamber with every intake stroke, no matter what the power setting is. In this sense, where spark ignited engines vary the intake charge (and compression ratio) - maintaining a constant air/fuel ratio, compression ignited engines vary the air/fuel ratio - not the intake charge.

Listen carefully next time you hear a diesel vehicle accelerate, then coast. They are quite loud with that traditional diesel "knock" during acceleration, but the combustion becomes almost inaudible during deceleration. This is because diesels do not need to maintain a constant air/fuel ratio in the cylinders, and can basically cut the fuel entirely when none is needed. DFCO (deceleration fuel cutoff) is an integral part of compression ignition engine operation. If you gun a diesel engine from idle, you'll hear it rev up loud, then wind down very quietly until it reaches the set idle speed, then sort of "whoosh" back into steady-state idle.

There is a limit to how lean of a mixture spark-ignited engines can run because beyond certain air/fuel ratio (somewhere around 18:1 for gasoline, I think) the fuel molecules are simply too sparse in the air charge to fully combust from being ignited by a spark. This is not a problem in the compression-ignition engine because the intake charge is always hot enough to ignite any amount of fuel.

So, from what I gather, it's the spark-ignition which is the limiting factor preventing gasoline engines from being able to direct-inject like diesels do. A diesel engine is actually closer in operation to a steam engine, partly because the higher energy content of fuel requires more heat and burns slower than gasoline does. In regards to diesel fuel injection, the duration of injection is one of the main factors that determines the amount of power generated in the cylinder, very similar to a steam engine.

As far as the compression discussion is concerned, I'm no expert, but my normally aspirated gasoline engine gives me more miles per gallon accelerating at only 75% load than at 100% load. Perhaps this is simply due to the design of the engine and transmission. But since the compression ratio is lower at lower throttle settings, due to smaller intake air charge, this would mean that my engine is more efficient at a lower-than-maximum compression ratio. Just my thoughts...
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Old 05-18-2011, 12:32 PM   #29 (permalink)
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Quote:
Originally Posted by abogart View Post
Getting back to the direct injection discussion (if you don't mind me rewinding the thread a bit here), the reason that diesels can run lean mixtures more easily is because the intake air charge is compressed to a temperature that is above the autoignition temperature of the fuel.

Since diesels are essentially unthrottled, a full air charge enters the combustion chamber with every intake stroke, no matter what the power setting is. In this sense, where spark ignited engines vary the intake charge (and compression ratio) - maintaining a constant air/fuel ratio, compression ignited engines vary the air/fuel ratio - not the intake charge.

Listen carefully next time you hear a diesel vehicle accelerate, then coast. They are quite loud with that traditional diesel "knock" during acceleration, but the combustion becomes almost inaudible during deceleration. This is because diesels do not need to maintain a constant air/fuel ratio in the cylinders, and can basically cut the fuel entirely when none is needed. DFCO (deceleration fuel cutoff) is an integral part of compression ignition engine operation. If you gun a diesel engine from idle, you'll hear it rev up loud, then wind down very quietly until it reaches the set idle speed, then sort of "whoosh" back into steady-state idle.

There is a limit to how lean of a mixture spark-ignited engines can run because beyond certain air/fuel ratio (somewhere around 18:1 for gasoline, I think) the fuel molecules are simply too sparse in the air charge to fully combust from being ignited by a spark. This is not a problem in the compression-ignition engine because the intake charge is always hot enough to ignite any amount of fuel.

So, from what I gather, it's the spark-ignition which is the limiting factor preventing gasoline engines from being able to direct-inject like diesels do. A diesel engine is actually closer in operation to a steam engine, partly because the higher energy content of fuel requires more heat and burns slower than gasoline does. In regards to diesel fuel injection, the duration of injection is one of the main factors that determines the amount of power generated in the cylinder, very similar to a steam engine.

As far as the compression discussion is concerned, I'm no expert, but my normally aspirated gasoline engine gives me more miles per gallon accelerating at only 75% load than at 100% load. Perhaps this is simply due to the design of the engine and transmission. But since the compression ratio is lower at lower throttle settings, due to smaller intake air charge, this would mean that my engine is more efficient at a lower-than-maximum compression ratio. Just my thoughts...
Old Mechanic brought it up first, here's an article on the Mazda Sky Activ engine (s).


Mazda SKYACTIV-G 1.3 Engine Details | Motorward

The gasser is direct injected.
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Old 05-18-2011, 12:51 PM   #30 (permalink)
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Hi abogart,

Excellent post, but a clarification:

Quote:
As far as the compression discussion is concerned, I'm no expert, but my normally aspirated gasoline engine gives me more miles per gallon accelerating at only 75% load than at 100% load.
I don't know if you mean 75% of full throttle at high rpm or wot at 75% of red line or something else. It's very likely that the engine is most efficient near wot and very near the rpm for peak torque.

-mort

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