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Old 12-25-2012, 12:01 AM   #11 (permalink)
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DI injector current shows injector timing relative to TDC

injection timing does not take place after top dead center in VW in DI engines , but under light load spark does
see in RED injector currents (like multi strike 2 per cylinder at this load),
see in Green spark signal IGT for cyl 1 from the ECM and
see in BLUE cylinder pressure , the two blue peaks show TDC 720 degrees apart
2 complete crankshaft revolutions ,
later in the same capture ,spark occurs well after TDC as the engine warms up


it takes time for the flame front to build / propagate and the increase in pressure is a part of that variable

in this capture injection never happens AFTER TDC even at WOT , this shows 1200 rpm from cool start ---
====================
EGR does NOT increase combustion temperature OR combustion pressure
EGR when active reduces BOTH combustion temperature and pressure
EGR does kinda sorta reduce compression ratio when active ( although many will say it does not ) by reducing the amount of air fuel mixture that can be drawn into the combustion chamber
by dilution with an INert gas - exhaust -

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Old 12-25-2012, 12:56 AM   #12 (permalink)
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Quote:
Originally Posted by mwebb View Post
EGR does NOT increase combustion temperature OR combustion pressure
EGR when active reduces BOTH combustion temperature and pressure
EGR does kinda sorta reduce compression ratio when active ( although many will say it does not ) by reducing the amount of air fuel mixture that can be drawn into the combustion chamber
by dilution with an INert gas - exhaust -
[/COLOR]
Sorry, I probably should've said "pre-ignition temperature". EGR increases the mass of gas in the cylinder, which increases the pressure and temperature before ignition occurs. That's not saying that it helps combustion at all, or that it increases combustion temperature, etc. Well, the charge certainly starts at a higher temperature but since it's diluted it ends at a lower temperature most likely.

Last edited by serialk11r; 12-25-2012 at 01:03 AM..
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Old 12-25-2012, 01:29 AM   #13 (permalink)
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Ya know a few weeks ago we went around about the variable valve timing, that essentially is a variable displacement method.

But....at the end of the day....all we are trying to cut out of the equation is the pumping losses, which on their worst day are costing us 4% efficiency. I came away thinking the variable valve timing was a pretty good solution.

60% of the energy losses in the engine are due to thermal losses. What can we do to even get 1/3 of that back? Pretty much double our mileage if you could.
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Old 12-25-2012, 01:48 AM   #14 (permalink)
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Quote:
Originally Posted by ChazInMT View Post
Ya know a few weeks ago we went around about the variable valve timing, that essentially is a variable displacement method.

But....at the end of the day....all we are trying to cut out of the equation is the pumping losses, which on their worst day are costing us 4% efficiency. I came away thinking the variable valve timing was a pretty good solution.

60% of the energy losses in the engine are due to thermal losses. What can we do to even get 1/3 of that back? Pretty much double our mileage if you could.
Uh wait where are you getting 60% from?

Pumping losses aren't a big deal at the end of the day like you say, but friction is! Friction eats away several times more energy than pumping. When that's considered, the actual heat cycle itself isn't doing too bad at maybe 50% efficiency. Could be bumped up to 60% with higher expansion ratio and multiple ignition points or something, but cooling losses are hard to solve when the engine is car sized and material constraints (and those emissions regulations) limit the temperatures we can burn fuel at. A second stage heat recovery solution (TEG, steam turbine, etc.) can get some of it back but the economics isn't super great.

Last edited by serialk11r; 12-25-2012 at 02:00 AM..
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Old 12-25-2012, 06:36 AM   #15 (permalink)
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I read somewhere that Saab had built an engine with an electric supercharger, nothing new there but seems like a viable thing to me.
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Old 12-25-2012, 09:03 AM   #16 (permalink)
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Friction losses vary as the rpm squared, twice the RPM, 4 times the friction loss. Variable valve timing does more then reduce pumping losses, it allows for more expansion on the combustion cycle. A small low speed turbocharged VVT lean burn adiabatic Atkinson cycle engine would be most efficient, probably running on alcohol, and put into a very light car.
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Old 12-25-2012, 09:23 AM   #17 (permalink)
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A question, how much heat energy is a Hypermiler like Pale Melanasian loosing in his Honda when he is averaging over 80 MPG? At EPA ratings the heat losss would be 60% to as high as 67%, but when you see what can be accomplished through Hypermiling then that same heat loss has been converted into movement of his vehicle.

If technique can reduce heat loss by say 50% then that 60% becomes 30%, not due to inefficiency in the engine, but due to inefficiency in the vehicle as a system. With sustained power requirements at less than 20% of available capacity, waste is inevitable.

I also think a lot of losses attributed to friction are losses due to reciprocation of the mass of pistons, a portion of the mass of connecting rods, hydraulic losses in oil circulation, as well as cam drive losses. All of these losses are greatly reduced by Hypermiling which minimises inefficient engine operation. Is it really the fault of the engine when you incorporate it into a poorly designed system. To me it's more like running your hot water all the time so you will have some hoit water when you need it, without waiting for the water to warm up.

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Old 12-25-2012, 04:17 PM   #18 (permalink)
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ummm no not really

Quote:
Originally Posted by serialk11r View Post
Sorry, I probably should've said "pre-ignition temperature". EGR increases the mass of gas in the cylinder, which increases the pressure and temperature before ignition occurs. That's not saying that it helps combustion at all, or that it increases combustion temperature, etc. Well, the charge certainly starts at a higher temperature but since it's diluted it ends at a lower temperature most likely.
EGR when active does not change the mass of the contents of the combustion chamber .

EGR dilutes the air fuel mixture with an INert gas, exhaust , total mass of the gas in the combustion chamber is UN changed or very slightly DEcreased
Temperature and therefore pressure are DECREASED (in the combustion chamber) when EGR is active

because EGR dilutes / displaces some the air fuel mixture
EGR effectively
DECREASE the volume of the combustion chamber which also
DECREASES engine displacement at the time by the percentage of EGR gas in the combustion chamber

so
the use of EGR is one very simple easy and currently used method of
creating
a

Variable Displacement Engine , which just happens to be
the topic of this thread
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Old 12-26-2012, 01:27 AM   #19 (permalink)
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Quote:
Originally Posted by ever_green View Post
i think i read somewhere that someone has already planning for a variable compression car and has already developed a prototype. i think it was saab, not sure...
SAAB developed a kind of oil-filled head gasket which led to the compression variation, but it was quite a complicated design, and even more complicated due to the valvetrain. Would be easier to apply to a 2-stroke design due to the absence of a valvetrain, altough a turbocharger with overbooster would be a more cost-effective option.
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Old 12-26-2012, 04:29 AM   #20 (permalink)
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Quote:
Originally Posted by mwebb View Post
EGR when active does not change the mass of the contents of the combustion chamber .

EGR dilutes the air fuel mixture with an INert gas, exhaust , total mass of the gas in the combustion chamber is UN changed or very slightly DEcreased
Temperature and therefore pressure are DECREASED (in the combustion chamber) when EGR is active

because EGR dilutes / displaces some the air fuel mixture
EGR effectively
DECREASE the volume of the combustion chamber which also
DECREASES engine displacement at the time by the percentage of EGR gas in the combustion chamber

so
the use of EGR is one very simple easy and currently used method of
creating
a

Variable Displacement Engine , which just happens to be
the topic of this thread
You have to consider at what time in the cycle. Before ignition, the pressure and temperature are higher because a greater mass (or number of particles or whatever you want) of gases (at approximately the same temperature) has been compressed to the same volume. After ignition, the same amount of heat energy from the fuel is going into a greater mass of gas, so the temperature is lower.

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