Will DOD be a DUD?

[4Vman]

With all this talk about "DOD" being the new catch phrase or acronym of the trendy for Fuel economy im not at all convinced
Im sure the car companies have done their homework however there are some points that concern me:
1) shutting off cylinders will create uneven heat production through the block, will this cause premature head gasket failure from warping?
2) will uneven heat distribution cause eratic expansion and contraction in the block causing uneven bore wear?
3) If a car requires 100kw's to maintain a constant speed does it matter how many cylinders it uses to make that 100kw's? Remember fuel has a caloretic value thats constant. eg: more power needs more fuel.
4) is there a tangiable benifit from producing 100kw's from 4 cylinders V 8 given frictional losses will be almost the same?

The early feedback im seeing from motors using this technology is there is very little gained in fuel economy in real world situations and the potential down sides as mentioned might be worse...

[EA2BA]

Without knowing precisely how it works, it would be possible to alternate the shut off cylinders hence keeping heat even, with no major negative effect.

[seano14]

Originally Posted by Dellboy999

Without knowing precisely how it works, it would be possible to alternate the shut off cylinders hence keeping heat even, with no major negative effect.

not to mention even engine wear too

I seem to recall it saving something like 5-10% in fuel. Its ok but it still takes a certain amount of fuel to keep a car moving, weight reduction and better gearing is better.

[flappist]

I have a "relative displacement on demand" engine right now......

[4.9 EF Futura]

Originally Posted by 4Vman

With all this talk about "DOD" being the new catch phrase or acronym of the trendy for Fuel economy im not at all convinced
Im sure the car companies have done their homework however there are some points that concern me:
1) shutting off cylinders will create uneven heat production through the block, will this cause premature head gasket failure from warping?
2) will uneven heat distribution cause eratic expansion and contraction in the block causing uneven bore wear?
3) If a car requires 100kw's to maintain a constant speed does it matter how many cylinders it uses to make that 100kw's? Remember fuel has a caloretic value thats constant. eg: more power needs more fuel.
4) is there a tangiable benifit from producing 100kw's from 4 cylinders V 8 given frictional losses will be almost the same?

The early feedback im seeing from motors using this technology is there is very little gained in fuel economy in real world situations and the potential down sides as mentioned might be worse...

Hey you raise some great points there. i.e. have 8 cylinders making 100kw or 4 cylinders working twice as hard and using just as much fuel to make same power.

One thing i have wondered - how much energy is used swinging pistons up and down and moving valves in and out on a cylinder which is not contributing any energy to the energy? Isnt this dead weight that's hindering the cylinders which remain active???

I guess the only real advantage I can see is having cylinders shut off at idle or when there is practically no load on the engine.

[4Vman]

Originally Posted by 4.9 EF Futura

Hey you raise some great points there. i.e. have 8 cylinders making 100kw or 4 cylinders working twice as hard and using just as much fuel to make same power.

One thing i have wondered - how much energy is used swinging pistons up and down and moving valves in and out on a cylinder which is not contributing any energy to the energy? Isnt this dead weight that's hindering the cylinders which remain active???

I guess the only real advantage I can see is having cylinders shut off at idle or when there is practically no load on the engine.

Yes exactly right, thats a point i haven't thought of either, the drag effect of "dead non contributing" cylinders.
I cant think of too many occasions during the day where DOD would actually work except if i went on a trip where i maintained constant speed for a long period or sat idle at the lights, the rest of the time the engine is either under load or braking so in a practical city driving application id thenk the advantages would be very marginal....

[xbgs351]

Originally Posted by 4.9 EF Futura

Hey you raise some great points there. i.e. have 8 cylinders making 100kw or 4 cylinders working twice as hard and using just as much fuel to make same power.

That is not true, as an engine is most efficient in terms of power produced per unit of energy, when at or near wide open throttle (WOT). Hence the 'smaller' motor is closer to WOT and will be more efficient.

[LPG EF2 GLI]

Originally Posted by xbgs351

That is not true, as an engine is most efficient in terms of power produced per unit of energy, when at or near wide open throttle (WOT). Hence the 'smaller' motor is closer to WOT and will be more efficient.

I'm pretty sure the ECU won't choose to run on 4 cylinders at wide-open throttle. Otherwise the system really is a DUD.

[xbgs351]

Originally Posted by LPG EF2 GLI

I'm pretty sure the ECU won't choose to run on 4 cylinders at wide-open throttle. Otherwise the system really is a DUD.

Your missing the point. The more throttle an engine gets the more efficient it is. Hence a motor running on half its cyclinders will be using more throttle and hence be more efficient.

[madmelon]

Originally Posted by xbgs351

Your missing the point. The more throttle an engine gets the more efficient it is. Hence a motor running on half its cyclinders will be using more throttle and hence be more efficient.

This is exactly right. The DOD would likely use the electronic throttle control to open the throttle slightly as the cylinders are deactivated. This allows those cylinders to achieve a higher "effective compression ratio" and so be more efficient than running all cylinders at a lower throttle opening.

When I say "effective compression ratio", I'm not talking about when the intake valve closes during the cycle, I'm talking about the fact that while an engine might be 10:1 compression, the fact that the air is restricted from entering (throttled) causes a decompression before it is then compressed in the cylinder. A 10:1 compression ratio may only give 2:1 or 3:1 at low throttle openings when viewed in this manner. As compression ratio is a huge factor in overall efficiency (research thermodynamic heat cycles, eg carnot) wider throttle opening means higher efficiency.

[Perana]

Yeah a lighter right foot helps a lot with fuel consumption!

IMO even though there is a slight gain by using this there are a lot of other technologies that manufacturers should be developing/adopting which in some cases not only reduce fuel usage but increase performance!

[Steffo]

Audi has had Displacement on Demand (they didn't call it this) in a few of their six-cylinder engines for quite a number of years now. They have deemed it crap technology, and therefore keep it out of their high end vehicles, and are phasing it out all-together. Its only becoming a hype now that two mainstream high-scale production companies are taking it up (GM and Chrysler).

The A4 3.0 V6's shut down 3 cyl during light throttle applications and city driving, and re-engage them when you give them some throttle... and it lags for 2 - 3 sec before putting the power on, horrible for overtaking etc. And the fuel saving benefits aren't that great either.

[73gscoupe]

Bugger DOD. Check saabs variable compression system out. you can have a normal na high compression motor, and a low compression high boost motor, and everthing inbetween:

http://www.saabnet.com/tsn/press/000318.html

order mine without the reduced engine displacement, just more turbo boost please...

Also electronic valve control. next step from VCT.

http://rbowes1.11net.com/dbowes/

Imagine full control of the valves over all engine RPM, load, throttle, etc. Still has a long way to go, but it just might get there.

Some of these systems (also think hybrid cars) have a high initial investment cost, but as fuel prices go up, they will become more and more viable.

DOD is really just a interim low capital outlay for low fuel savings measure.

[parawolf]

The problems also exist in how random is the piston/cylinder shutdown? Do they always shutdown the same cylinders? If so then there is going to be heat issues without a doubt. Other reasons why they don't get such a massive saving in petrol is that they still have to spray petrol (smaller amounts given) into the cylinder to ensure lubrication.

I have my concerns about this being a silver bullet. A better designed engine to match with what the user is going to do with the car vs using a 'catch-all' engine design in a RWD sports car, to a 4WD, to a FWD.

[4Vman]

Originally Posted by parawolf

The problems also exist in how random is the piston/cylinder shutdown? Do they always shutdown the same cylinders? If so then there is going to be heat issues without a doubt. Other reasons why they don't get such a massive saving in petrol is that they still have to spray petrol (smaller amounts given) into the cylinder to ensure lubrication.

I have my concerns about this being a silver bullet. A better designed engine to match with what the user is going to do with the car vs using a 'catch-all' engine design in a RWD sports car, to a 4WD, to a FWD.

Yes, lubricating a dead cylinder would require a bit of fuel, aso combustion pressure helps hold the rings in place to reduce oil consumption... too many potential issues for my liking..
Does anyone remember ELB (extra lean burn) in the chryslers? with the lights on the front guards and vaccuum guage on the dash?? it was a failure because it leaned out cylinders and caused detonation and over heat issues..

[xbgs351]

Originally Posted by 4Vman

Yes, lubricating a dead cylinder would require a bit of fuel, aso combustion pressure helps hold the rings in place to reduce oil consumption... too many potential issues for my liking..
Does anyone remember ELB (extra lean burn) in the chryslers? with the lights on the front guards and vaccuum guage on the dash?? it was a failure because it leaned out cylinders and caused detonation and over heat issues..

What do you mean by lubricating cylinder with fuel?

[Bossxr8]

Originally Posted by parawolf

The problems also exist in how random is the piston/cylinder shutdown? Do they always shutdown the same cylinders? If so then there is going to be heat issues without a doubt. Other reasons why they don't get such a massive saving in petrol is that they still have to spray petrol (smaller amounts given) into the cylinder to ensure lubrication.

I have my concerns about this being a silver bullet. A better designed engine to match with what the user is going to do with the car vs using a 'catch-all' engine design in a RWD sports car, to a 4WD, to a FWD.

I'm pretty sure its rotated through a different number of cylinders. The fail safe cooling system on the BA shuts down some cylinders and rotates them through a complete cycle.

[Psycho Chicken]

I can't see much of a point, the piston on the power stroke has to work twice as hard. You're still running compression strokes, which rob power. Unless there's some sort of VTEC type cam lobe that bleeds compression out the exhaust or something? Even still, friction would be high.

[Steve D]

Originally Posted by Psycho Chicken

I can't see much of a point, the piston on the power stroke has to work twice as hard. You're still running compression strokes, which rob power. Unless there's some sort of VTEC type cam lobe that bleeds compression out the exhaust or something? Even still, friction would be high.

As I understand it, the 'inactive' cylinders act more like a spring. You've got to use energy to compress it, but you get that energy back on the expansion part of the stroke, so you're just left with friction losses only.

Originally Posted by 4Vman

If a car requires 100kw's to maintain a constant speed does it matter how many cylinders it uses to make that 100kw's? Remember fuel has a caloretic value thats constant. eg: more power needs more fuel.

I'd doubt you need that much power to maintain a constant speed. Most of the time, you're on part throttle and using only a small fraction of your engine's potential. The only times your engine's really working hard is under heavy acceleration, travelling near Vmax or during a dyno test.

[Sox]

Originally Posted by Steve D

As I understand it, the 'inactive' cylinders act more like a spring. You've got to use energy to compress it, but you get that energy back on the expansion part of the stroke, so you're just left with friction losses only.

How do you get the energy back if there is no combustion?
The must be some kind of valve deactivation for there to be no upstroke compression losses.

I'd doubt you need that much power to maintain a constant speed. Most of the time, you're on part throttle and using only a small fraction of your engine's potential. The only times your engine's really working hard is under heavy acceleration, travelling near Vmax or during a dyno test.

I suspect he used 100kw as an example, his reasoning still applies.

Rick.

[4Vman]

Originally Posted by Sox

How do you get the energy back if there is no combustion?
The must be some kind of valve deactivation for there to be no upstroke compression losses.

I suspect he used 100kw as an example, his reasoning still applies.

Rick.

Yes, spot on, it was a theoretical example.

[Steve D]

Form an Automotive Industries artcile (US trade mag):

Unlike GM’s system, which uses an add-on oiling plenum to fill the lifters, the Hemi’s hydraulic lifters are fed through oil passages machined into the block. With the use of Electronic Throttle Control and sophisticated algorithms the transition from four to eight cylinders can be done in 40 ms.

As for the second point, this would probably been a better quote to bounce off:

Originally Posted by 4.9 EF Futura

Hey you raise some great points there. i.e. have 8 cylinders making 100kw or 4 cylinders working twice as hard and using just as much fuel to make same power.

Getting 100kW out of, say, a 240kW V8 would probably require all cylinders to be working, but if only 50kW is necessary to maintain 100km/h, then it's less of an issue.

Mind you, I'd probably wait a while before buying one myself!

[Fairlane]

I think it will be a dud, that being said if someone offered me a hemi 300c, i wouldnt argue.

I think we should however be looking at better auto transmissions (ie our new BF box), DSG transmissions, newer lower friction materials and of course a bit of weight reduction.

[Cypher]

I like what Saab has done, this will be the future of petrol engines as fuel costs go up and fuel eventually runs out.

What about alternative fuels? Maybe we can still keep the existing engines if we can find a cheap fuel alternative to petrol.

[4Vman]

Originally Posted by Cypher

What about alternative fuels? Maybe we can still keep the existing engines if we can find a cheap fuel alternative to petrol.

Resist Steffo, RESIST!!!! :

[Iphido]

DOD is a marginal improvement. Its not perfect and its not a cure all.

It seems theres more to be gained with Direct injection technology. Its also easier to impliment on current engines.

GM seems to be reconcidering after their DOD system seemed to create more problems than it solved.

[ssj_jaypee]

I wonder if dual camshafts could be considered a possibility in these circumstances.
Similar to car companies like Lotus, the first cam is used whilst driving under a certain rev range to improve fuel economy, put the foot down the second cam engages and the power comes on.

[Psycho Chicken]

That's what VTEC is.

[ssj_jaypee]

Good stuff. Why not do a V8-TEC then?