The reason diesels are efficent at idle? Solved! - Page 3 - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

Go Back   Fuelly Forums > Fuel Talk > Diesels
Search Today's Posts Mark Forums Read Click Here to Login
Reply
 
Thread Tools Display Modes
 
Old 11-01-2009, 01:09 PM   #21
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
Quote:
Originally Posted by milesgallon.com View Post
The high pressure makes the temperature in the cylinder so high that the fuel will ignite by itself as soon as it's sprayed into the cylinder.
Sounds like new direct injection gasoline engines may get approximately the same benefit.
__________________

__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Old 11-01-2009, 02:23 PM   #22
Registered Member
 
GasSavers_bobski's Avatar
 
Join Date: May 2007
Posts: 463
Country: United States
Quote:
Originally Posted by theholycow View Post
Sounds like new direct injection gasoline engines may get approximately the same benefit.
Sort of. You get the similar benefit of being able to ignore the air charge that isn't needed, but direct injection doesn't rely on autoignition - it still uses a spark plug. In that respect, I think it has more in common with stratified charge engine designs than diesel.

The problem diesels have is with getting all the air/fuel to burn, and quickly enough to make efficient use of the released energy (which incidentally is one of the reasons diesels typically have low redlines). In that respect, gasoline direct injection has an advantage. GDI can spray a small amount of fuel into the pocket around the spark plug, ignoring the surrounding unneeded air to get diesel-like operation. Or, to get typical gasoline engine peak power output, it can inject fuel during the intake and compression strokes, evenly pre-mixing the air and fuel without worrying about the mixture auto-igniting before it is mechanically advantageous (aka pre-ignition) as would happen if you tried the same thing in a diesel engine. It could also operate at points in between those two extremes (mimicking stratified charge engine operation) by providing a leaned-out premix and tossing in a quick burst of additional fuel around the plug to light it off.
__________________

GasSavers_bobski is offline   Reply With Quote
Old 11-02-2009, 02:27 PM   #23
Registered Member
 
milesgallon.com's Avatar
 
Join Date: Mar 2007
Posts: 39
Country: United States
Are there any direct injection gasoline engines used in real mass production cars yet?

Simon
__________________
Handy gas mileage and automotive calculators at MilesGallon.com lets you find out the real mpg of your vehicle, calculate the fuel consumption and cost for a trip and how about a tool that gives you the total cost per mile for a specifice vehicle over its lifetime.

milesgallon.com is offline   Reply With Quote
Old 11-02-2009, 02:37 PM   #24
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
Yes, plenty of them. GM has been putting them in the Cadillac CTS for a few years, and the same engine is in the V6 Camaro. I think the Ecotec I4 is too.

Volkswagen's 2.0 FSI-series engines are DI.

I think most manufacturers have at least one DI gas engine.
__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Old 11-02-2009, 03:55 PM   #25
Registered Member
 
JanGeo's Avatar
 
Join Date: Jan 2006
Posts: 2,442
Country: United States
Send a message via Yahoo to JanGeo
Landspeed - one thing you didn't get on the engine braking with the ignition off is when you opened up the throttle you got that the intake valve closes at the bottom of the stroke and compresses the gas all the way to TDC then the plug would fire and you should get all the compression effort back on the down stroke BUT you forgot that the exhaust valve opens BEFORE the piston goes very far down the power stroke so it ends up dumping the compressed air out the eshaust before the compressed air can put that compression energy back into the piston stroke. That is why giving it more throttle didn't reduce engine braking effect - it can in engines with later exhaust valve openings like my Old 65 Rambler American Flat Head 6 however.
JanGeo is offline   Reply With Quote
Old 11-02-2009, 04:22 PM   #26
Registered Member
 
GasSavers_bobski's Avatar
 
Join Date: May 2007
Posts: 463
Country: United States
Quote:
Originally Posted by JanGeo View Post
BUT you forgot that the exhaust valve opens BEFORE the piston goes very far down the power stroke so it ends up dumping the compressed air out the eshaust before the compressed air can put that compression energy back into the piston stroke. That is why giving it more throttle didn't reduce engine braking effect - it can in engines with later exhaust valve openings like my Old 65 Rambler American Flat Head 6 however.
I think you've got that backwards. When the exhaust valve opens, pressure in the combustion chamber drops, so the force pressing against the piston disappears; The energy stored as compressed air is wasted. The sooner in the power stroke the valve opens, the less energy is recovered. Going to one extreme, if the exhaust valve opened at the beginning of the power stroke so that no energy was recovered, the degree of engine braking would be entirely dependent on how much work it takes to push the piston to the top of the cylinder during the compression stroke. The higher the initial air pressure in the cylinder (dependent on manifold pressure and thus throttle position), the greater the effort it will take to compress it.
GasSavers_bobski is offline   Reply With Quote
Old 11-03-2009, 06:09 PM   #27
Registered Member
 
JanGeo's Avatar
 
Join Date: Jan 2006
Posts: 2,442
Country: United States
Send a message via Yahoo to JanGeo
NO I got it right just like you said bobski - the exhaust opens too soon letting the pressure out so it can't continue pushing the piston down to recover the energy.
JanGeo is offline   Reply With Quote
Old 11-03-2009, 07:09 PM   #28
Registered Member
 
GasSavers_bobski's Avatar
 
Join Date: May 2007
Posts: 463
Country: United States
Quote:
Originally Posted by JanGeo View Post
NO I got it right just like you said bobski - the exhaust opens too soon letting the pressure out so it can't continue pushing the piston down to recover the energy.
Right... I'm saying your conclusion about later exhaust valve openings increasing the throttle-modulated engine braking effect is backwards. I guess I could have cropped the quote a little better.
GasSavers_bobski is offline   Reply With Quote
Old 11-29-2009, 06:11 PM   #29
Registered Member
 
DarbyWalters's Avatar
 
Join Date: Oct 2007
Posts: 376
Country: United States
Problem is Diesels do not really have throttle plates...they jump pump air unrestricted. You acceleration is solely thru addition of diesel fuel. That is why Diesels idle so efficiently. Air to Fuel Ratios can be VERY LEAN.
__________________
2006 Jeep Liberty CRD...Founder of L.O.S.T.
OME 2.25" Lift w/ Toyo Open Country HTs 235/75/16s
ASFIR Alum Eng/Tranny/Transfercase/Fuel Skids
2002 Air Box Mod...Air Tabs (5) on Roof...(3)each behind rear windows
Partial Grill Block with Custom Air Scoop and 3" Open Catback Exhaust
Lambretta UNO150cc 4 Stroke Scooter



DarbyWalters is offline   Reply With Quote
Old 04-24-2013, 03:24 PM   #30
Registered Member
 
Join Date: Apr 2013
Posts: 1
Country: United States
Actually, if it makes any difference, opening the throttle plate should increase the braking effect. When coasting, the engine is functioning as a pump. The pumping losses are the energy used to pump air through the engine. This happens under power, too, but under power the energy produced by the expansion of gas in the combustion chamber overpowers the pumping losses. In a pump, the energy drawn by the pump is positively related to the amount of fluid pumped. Hence, opening the throttle plate will increase the resistance. To really reduce the pumping losses, some engines close off the valves of some of the cylinders. With no air going through those cylinders, there are no pumping losses at those cylinders.
__________________

rh13 is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Lost three fills! lovemysantafe Fuelly Web Support and Community News 1 11-25-2011 10:41 PM
Make the page lengths 30 posts again, please SVOboy Fuelly Web Support and Community News 4 05-18-2006 01:43 PM

Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 08:59 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.