Will DOD be a DUD?

4Vman · 22-09-2005, 03:36 PM

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 · 22-09-2005, 03:39 PM

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.

22-09-2005, 03:45 PM

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.

seano14 · 22-09-2005, 03:46 PM

Quote:

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

flappist · 22-09-2005, 04:00 PM

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

4.9 EF Futura · 22-09-2005, 04:09 PM

Quote:

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.

Perana · 22-09-2005, 04:11 PM

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 · 22-09-2005, 04:12 PM

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.

4Vman · 22-09-2005, 04:13 PM

Quote:

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....

parawolf · 22-09-2005, 04:25 PM

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 · 22-09-2005, 04:29 PM

Quote:

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..

73gscoupe · 22-09-2005, 04:55 PM

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.

Bossxr8 · 22-09-2005, 06:18 PM

Quote:

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 · 22-09-2005, 06:24 PM

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.

Fairlane · 22-09-2005, 07:14 PM

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 · 22-09-2005, 07:30 PM

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 · 22-09-2005, 07:33 PM

Quote:

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 · 22-09-2005, 07:44 PM

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 · 22-09-2005, 07:54 PM

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 · 22-09-2005, 07:56 PM

That's what VTEC is.

ssj_jaypee · 22-09-2005, 07:57 PM

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

OzJavelin · 22-09-2005, 08:51 PM

Caddy tried DoD in the early eighties and it was a dud .. the technology didn't exist to get it working right. The new system is probably a lot better, but as mentioned earlier, the savings are only up to 20% max. Probably not bad though when most cars sit idling in city traffic?

We'll see how good it is when the 300C shows up here .. remember Mother Mopar was an innovator in America (post 50s), so hopefully it's not all hype.

noosacuda · 23-09-2005, 08:50 AM

As Ozjavelin said Cadillac tried it in the early 80's with the V8-6-4,it didn't work then but it should work now with modern computers etc in the new cars. Look how turbos have progressed from the 60's Oldsmobile to modern Rice cars? I owned an '81 Turbo Trans-Am, (301 V8) it didn't work that well because the technology wasn't available at the time.(Btw Chryslers system was Electronic Lean Burn)

parawolf · 23-09-2005, 09:49 AM

Quote:

Originally Posted by ssj_jaypee

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

Not really that pratical. When you look at the VVTi-L that is in the Lotus Elise/Exige and Toyota Corolla Sportivo and Celica you notice that the readline is all the way up at 8250rpm or there abouts.

For a 'commodity' V8 like a BOSS or LS2, There is no way such a big engine to going to run to those engine speeds. If you have ever driven a Lotus Elise 111R you will notice that yeah it has 'some' poke under 4500rpm (mostly due to the lightweight) but once it gets above 5000rpm the engine RPM headbuts the 8250rpm limiter REALLY quickly. This sort of technology is only any good in fast lightweight engines and lightweight chassis. Compare the Lotus to the Celica, same engine, and it is doughy as around town, but above 5000rpm and if you can keep it there its a lot of fun (just understeers at those speeds thanks to front wheel drive).

Because a BOSS or an LS2 doesn't rev to 8000+rpm in 'natural' configuration there is little point. You may as well go with some more 'generic' higher lift cam shafts, and suffer a bit at idle-1300rpm, and then you can get around that with high compression pistons or perhaps VCT (Barra 3V).

It just isn't required on the V8's. On a Aston Martin Vanquish or a Ferrari, possibly because the $$$ are there for the engineering to allow the engines to spin that fast. But you are hardly talking commodity economics then.

**Cav** · 23-09-2005, 10:14 AM

DOD be buggered, I have one simple request...

I want a large passenger car that has plenty of power, very good fuel economy, and sounds like a V8 with dual exhausts.

I ask you .... is that too much to ask?

PS ...and I look good driving it!

parawolf · 23-09-2005, 10:21 AM

Quote:

Originally Posted by Cav

DOD be buggered, I have one simple request...

I want a large passenger car that has plenty of power, very good fuel economy, and sounds like a V8 with dual exhausts.

I ask you .... is that too much to ask?

PS ...and I look good driving it!

So you want a falcon that has a 3Litre V6 Turbo Diesel?

**Cav** · 23-09-2005, 10:26 AM

Sounds interesting parawolf, as long as I looked good driving it of course

4Vman · 23-09-2005, 10:34 AM

lots of Power and very good fuel economy shouldn't really be mentioned in the same sentence, efficiency aside to make more power you need to burn more fuel......

parawolf · 23-09-2005, 10:38 AM

Quote:

Originally Posted by 4Vman

lots of Power and very good fuel economy shouldn't really be mentioned in the same sentence, efficiency aside to make more power you need to burn more fuel......

Depends on your defination of power.

Lots of power usually refers to horsepower or kW.

Torque provides you with the go, hence the Turbo Diesel

:

merlin · 23-09-2005, 10:43 AM

Quote:

Originally Posted by parawolf

Depends on your defination of power.

Lots of power usually refers to horsepower or kW.

Torque provides you with the go, hence the Turbo Diesel

:

Depends on your definition of "go". I don't think tractors or B-doubles have much "go" but they have plenty of torque. I'd rather have horsepower.