IDI - effect of reducing compression ratio on hp & mpg

[greg123]

Higher temps great, but they come from considerable resistance placed on the piston dring compression and friction losses driving the air through the swirl chamber entry hole.  Energy doesn't come for free and if we presume the same amount of fuel is burnt (eg same % of unburnt fuel in both cases) then the only power extracted is the same, from the explosion down stroke.  This would seem to me to indicate a 30:1 cr would drag more power off the engine, not increase it.  If a power gain is seen, it must be due to previous insufficient cr to generate enough heat to ignite the fuel properly, or not enough velocity on swirl to mix it - rather than a product of the pressure it's self.

Comments please ;-)

Well, if you have more power with the same fuel/air mixture, then you have greater efficiency. Higher temps mean greater pressure, meaning more force on the piston.

[tylernt]

I wish there was an easy way to experiment with C/R by using different glow plugs or something... a skinnier glow plug would be about as easy as it gets.

[Mark(The Miser)UK]

I think exploding the fuel gets the most energy out of the fuel but does not get the most work done.  Burning gives a more sustained push with the gases which works for longer on the crank. Ceramics of course would slow down the losses from higher heat.  The CAV set up shows how high injector pressure is not neccessary for an efficient burn. Like the early 1.5 VW diesels.

Greg, I nearly put a 1.6 TD in my mother-in-laws mk1 Polo! No brake servo put me off :lol:

[QuickTD]

It seems that the general consensus here is that compressing the air on the compression stroke represents a net energy loss. This isn't entirely true. At the end of the compression stroke, if no fuel is added, the air compressed within the combustion chamber will expand again and drive the piston back down, think of the air as a spring. The pressure in the cylinder will then be much the same as at the end of the intake stroke. Very little work is done and therefore the losses are minimal. The losses would consist of the heat lost during compression the some energy lost to moving the air through the intake and exhaust valves.

 Efficiency gains or losses due to raising/lowering the compression ratio should focus more on the dynamics of combustion than the negligable frictional losses due to compressing air. As far as I know, the higher the compression ratio, the higher the efficiency but the point of greatly diminishing returns occurs somewhere around 17:1. Higher compression ratios will very slightly increase efficiency but at the cost of greatly increased mechanical stress on the engine. Most automotive engine manufacturers use higher ratio's to improve starting and slow speed operation but not necessarily to increase efficiency. In the end, the factory compression ratio would probably be optimal. Slightly lower ratios will probably yield minor losses in efficiency.

[tylernt]

I wish there was an easy way to experiment with C/R by using different glow plugs or something... a skinnier glow plug would be about as easy as it gets.

Custom thicker heatshields?

Ooh, good idea. Or just some shims to shim up the existing heatshields. Using copper should still allow a crush seal.

You will be altering the spray pattern a bit though -- effectively widening it, I think. Hopefully not enough to make a difference.

[Black Smokin' Diesel]

Gasoline engines have an average compression ratio of 10:1, much lower than the 23:1 of a VW IDI diesel. The higher the compression on a gas car, the more prone it is to knocking, thus requiring high octane fuel or race fuel when a certain ratio is reached (12.5:1 and higher). Gasoline is much easier to ignite on compression alone, which is why gassers fear pre-detonation.

I will respectfully disagree. Gasoline is subject to accidental "knocking" at compression ratios higher than 10:1, but that does not mean it will ignite easily or reliably in a diesel. The major factor in gasoline engine preignition is the presence of the combustable mixture in the combustion chamber during the compression stroke. Diesels suffer no such "flaw", the fuel is introduced only at the desired moment of ignition, so pre-ignition is a non issue.

Good point. If we were to inject gasoline at the desired moment of ignition, it would work. IIRC, isn't it how FSI engines work? They have a pretty high injection pressure (there's between 435 and 1595psi of pressure in the fuel rail).

Efficiency gains or losses due to raising/lowering the compression ratio should focus more on the dynamics of combustion than the negligable frictional losses due to compressing air. As far as I know, the higher the compression ratio, the higher the efficiency but the point of greatly diminishing returns occurs somewhere around 17:1. Higher compression ratios will very slightly increase efficiency but at the cost of greatly increased mechanical stress on the engine. Most automotive engine manufacturers use higher ratio's to improve starting and slow speed operation but not necessarily to increase efficiency. In the end, the factory compression ratio would probably be optimal. Slightly lower ratios will probably yield minor losses in efficiency.

Isn't there a vw paper that states that the most efficient compression ratio for a diesel is 18:1 or 18.5:1? I remember fspGTD saying something about that.

I wish there was an easy way to experiment with C/R by using different glow plugs or something... a skinnier glow plug would be about as easy as it gets.

Custom thicker heatshields?

Ooh, good idea. Or just some shims to shim up the existing heatshields. Using copper should still allow a crush seal.

You will be altering the spray pattern a bit though -- effectively widening it, I think. Hopefully not enough to make a difference.

Some time ago I bought a gasket set for my 1.6TD. It came with thin heatshields. I didn't know the kit included heatshields so I ordere a set separately. The separate set was a lot thicker, maybe twice as thick and looked much more heavy duty. Unfortunatey I can't compare the two since the head the thin ones were in cracked so I replaced it along with the injectors and used the thick heatshields.

The 1.9TD metal headgasket is a nice OEM way to slightly decrease the compression ration as the squish are is greater due to the cylinder bores being larger. So now I guess my compression ratio is lower due to the thicker heatshields and the 1.9 HG.

[greg123]

No, amazingly a tank full of petrol in yoyr diesel doesn't run.  The 'direct injection' and high pressure petrols still squirt the stuff in during the intake stroke!  They still rely on a spark to ignite it.

Good point. If we were to inject gasoline at the desired moment of ignition, it would work. IIRC, isn't it how FSI engines work? They have a pretty high injection pressure (there's between 435 and 1595psi of pressure in the fuel rail).

[jtanguay]

doesn't the old diesel manuals state that you can add up to 50% gasoline in extremely cold cases???  the only real problem i see with gas is that it's not a good lubricant like diesel, and has less energy.  probably why the old trucks used to be able to be started on it, then switched over to the 'good stuff'

[greg123]

20%.  Lubrication longer term would be an issue, a more immidate issue is that petrol won't ignite if injected in a diesel motor ;-)

Old trucks didn't start on it in that sense, they ran as a petrol engine with high cr when cold and a diesel engine with low cr when hot.  When running as a petrol engine they had plugs and a carb, when running as a diesel the fuel was injected with a ip.  I think some people are also getting confused with the 'TVO' or diesel type (parrafin) tractors that were NOT diesel engines, a hot petrol engine could run on lower grade fuel (inbetween petrol and diesel) but wouldnt' start on it, so they started on petrol and when hot switched to TVO.  The whole system was run completely as a petrol engine, plugs and no IP.  It wasn't an amazing idea and hasn't been used much in modern times.  The most modern vehicle I worked on with that system was an old MF tractor form the early 50's I think.  I used a moped engine to test this system, it ran okay when hot on parrafin but wouldn't even start when hot on it, I had to switch back to petrol to get it to start even when hot.  I also had to alter the jetting for the parrafin running compared to petrol, which is understandable.

Greg.

doesn't the old diesel manuals state that you can add up to 50% gasoline in extremely cold cases???  the only real problem i see with gas is that it's not a good lubricant like diesel, and has less energy.  probably why the old trucks used to be able to be started on it, then switched over to the 'good stuff'

[greg123]

Been thinking on your idea, how about - for experimental purposes only - taking out the glow plugs completely and blocking the holes with bolt (set screw) and washer making sure the threads are only as long as needed, or for that matter a sawn off old set of glow plugs sawn off just past the seat.

It would be hard to start but I'm sure it could be got going by a bit of cranking/bump start/liquid in the manifold, once running it should be fine.  Quite how we would establish efficiency without then driving it 500 miles I'm not sure...  Unless you live on a hill and can put up with it for a week? ;-)

I wish there was an easy way to experiment with C/R by using different glow plugs or something... a skinnier glow plug would be about as easy as it gets.

[Black Smokin' Diesel]

It would be hard to start but I'm sure it could be got going by a bit of cranking/bump start/liquid in the manifold, once running it should be fine.  Quite how we would establish efficiency without then driving it 500 miles I'm not sure...  Unless you live on a hill and can put up with it for a week? ;-)

If the engine has good compression and a good battery, it'll start without additives. I was able to start my 1.6NA with three burnt glow plugs, in -40 weather, unplugged. I took a lot of cranking but it did start.

If you found a way to heat the intake air only during startup, I think it could help. Something like locating one or two GP on the intake manifold. The amount of space they would take in the manifold would be negligible compared to the space they take in the cylinder head.

BTW, disable HTML in your profile, that way your quotes will work properly :wink:

[lyeinyoureye]

This is gonna sound kinda nutty, but provided the owner lived in a warm enough climate, say... close to the equator, could an IDI start and run reasonably well with a ~16:1 CR? From everything I've gathered, it's not CR that'll make or break diesel performance/efficiency, but quench. Which is why a head gasket that's too tall can hurt an engine way more than the lower CR seen in the TDIs. From the 1.5L SAE paper, the main reason for a higher CR was cold starting concerns, but w/o those, how much farther could we go down in terms of CR?

Also, what about running a grid heater, pulling the glow plugs, installing spark plugs, and running E85 with TBI/MS&S above ~4-5k rpm? Diesels don't have the greatest air utilization rates, but there should be plenty for higher octane fuel and gasoline as long as there's spark. We could rev the engine a make plenty more power above ~4k rpm, while still maintaining the characteristics that make diesels efficient at low load.

[rabbid79]

I wish there was an easy way to experiment with C/R by using different glow plugs or something... a skinnier glow plug would be about as easy as it gets.

I've done the math.  Removing the glow plug completely only lowers the compression by about 0.5, so 23.5:1 becomes 23.0:1.  The volume of the glow plug is suprisingly little.

[greg123]

You'd think so wouldn't you?  However try that on a Peugeot 1.9TD, the things never start without glow plugs - even if hot!  Well they would eventually when hot but they do NOT like it.  My older Peugeot 2.5l idi in my motorhome is a worse design and dates back nearly 30 years, however it starts first swing and once it's been started for the rest of the day it doesn't need glow plugs.  So I think there is a bit more to play than just the cr, such as the designed energy of the swirl etc which may be low at low cranking rpm necessitating glow plugs for some engines - even when hot.

If the engine has good compression and a good battery, it'll start without additives. I was able to start my 1.6NA with three burnt glow plugs, in -40 weather, unplugged. I took a lot of cranking but it did start.

If you found a way to heat the intake air only during startup, I think it could help. Something like locating one or two GP on the intake manifold. The amount of space they would take in the manifold would be negligible compared to the space they take in the cylinder head.

BTW, disable HTML in your profile, that way your quotes will work properly :wink:

[greg123]

I was going to ask if anyone knew the volume ;-)  Okay that is not such a helpful idea then!

I've done the math.  Removing the glow plug completely only lowers the compression by about 0.5, so 23.5:1 becomes 23.0:1.  The volume of the glow plug is suprisingly little.