IDI - effect of reducing compression ratio on hp & mpg

[greg123]

I'm building a diesel special soon, not VW but very similar in many respects, AX 1.4 idi diesel (excellent engine, bad alloy block).  I'm using a petrol GTI cast iron block and thinking about putting the later 1527cc head on the 1360cc block, which will drop compression to 20:!, via a larger pre combustion chamber.

I also am thinking about full ceramic treatment.  I have read a research paper which showed that fuel economy of 5% better was achieved by dropping compression down to 15:1, of course starting in the cold may be hard but I'm not too concerned about that especially not at 20:!.

Wanted to start a theory & pratical discussion on lowering compression ratio, enlargin the pre chamber and *possibly* reduce pumping frictional losses by increasing the bore of the entry hole to the pre chamber cup, anyone messed with this stuff?

Oh by the way 1st stage is to see what mpg I can get from this motor, it did 71mpg in stock form before, I have several improvements to make and a couple on the vehicle as well (narrower taller tyres, better fluids, aero engine guard, electric brake vacuum pump if I can find one).  2nd stage may be a turbo to see how quick I can make it, so a low compression and coated engine would be good prep for that.

I don't want to confuse with leaking rings/valves/worn out low compression, I'm talking a 100% fit and fettled engine, but working on bigger pre-combustion chamber size which is a different thing to an engine that won't start because all the compression is coming out the breather, obviously that's a big waste of power!

Greg.

[jtanguay]

my polish mechanic talked about these russian trucks that would first start on gasoline, then switch over to diesel... they had lower compression than 15 i think...  i wonder how it worked though... gasoline isn't a good lubricator for the pump... so that would damage our engines.  but what about say 50/50 mix in order for it to ignite ?  then you would need a purging system once the vehicle is all warmed up...

if you can find a way to heat up the motor before each start you won't really have an issue.  the high compression ratio is mainly for cold starting.

at 15:1 compression ratio i think you might be able to use an m-TDI pump on the car...  of course you will need TDI injectors though... but the piston's might not work properly as TDI ones have special swirl chambers machined right inside...

71 mpg is great!!!  imo lower compression ratio will give better power/fuel economy...

[boosted_diesel_84]

my friend who is a farmer has a bunch of old tractors, many that are diesel, but start using gasoline, by having a type of bleed valve, where some compression is bleed out so the gas can ignite, it still detonates like a diesel, but then you switch over to the diesel pump by turning a valve, once its warmed up, so i know it can be done, especially if i think its a Farmall, that did it  at least 50 years ago.

just curious though, wouldnt a higher compression make it more thermally efficient? therefore be better on fuel?

[QuickTD]

Any gas/diesel combination engine I've witnessed has required the use of regular spark ignition to start on gasoline. These engines also use a regular carburator when running on gasoline. Once the combustion chamber was warmed sufficiently the diesel injection pump is activated and the carburator bypass valve is opened allowing free flow of air into the engine and it operates as a normal diesel.

 Gasoline has a much higher antiknock index than diesel, so it is much, much harder to ignite from the heat of compression than diesel fuel. The higher the octane of the gasoline, the less prone to ignition due to heat or pressure.

 In short, to start on gasoline you'll need a carburator and spark ignition, or a compression ratio much higher than a regular diesel, which isn't the point really...

 Getting back to the point, the compression ratio at which a diesel will reliably start depends alot on the surface area to volume ratio of the combustion chamber. Very large engines tend to start and run much better than small ones at low compression ratios due to the lower surface to volume ratio and the resultant lower heat losses to the combustion  chamber walls. Direct injection engines also start better than indirect injection engines, again due to more favorable surface to volume ratios. I believe the the 17:1 figure of optimum efficiency was achieved on one of recardo's very large, slow speed single cylinder test engines, not really the same animal as a modern small diesel.  

 A 1.5l 4 cylinder will probably start an run acceptably with a 20:1 compression ratio in a relatively warm climate. Andy2's car starts a bit poorly with ~18-18.5:1 compression ratio and I'm told lacks torque at low rpm and boost pressures. If you are going for ultimate economy, very low compression might not be the hot ticket. If you're going for power it is, the decrease in compression ratio lowers the cylinder pressure for a given boost pressure and the added boost more than makes up for the lack of compression, once the turbo(s) spool, of course.

 Modifications to the prechamber oriface are similarly a double edged sword. The hole needs to be small enough to insure good air velocity in the chamber during compression to aid in charge mixing, particularly at low engine speeds. It also must be large enough to minimize pumping losses. I would assume that larger orifaces would yield more gains in the upper range of RPM/power and smaller orifaces would perform better at low speeds. For an economy motor that will spend most of its time at low speeds the stock oriface is probably a good compromise.

[Black Smokin' Diesel]

Gasoline has a much higher antiknock index than diesel, so it is much, much harder to ignite from the heat of compression than diesel fuel. The higher the octane of the gasoline, the less prone to ignition due to heat or pressure.

 In short, to start on gasoline you'll need a carburator and spark ignition, or a compression ratio much higher than a regular diesel, which isn't the point really...

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.

Anyway, I don't think starting the engine on gasoline is a good option in this case, a lot of modding is required (bleed valve, carburator...).

[jtanguay]

well gasoline might not be the greatest option, but i'm sure there is something out there (other than quickstart...)

i thought that the higher compression ratio requires more work to achieve, therefore lower compression ratio relieves that stress... so it's more efficient to have higher compression ratio's?? why do race gas engine's use higher CR's then???

[Mark(The Miser)UK]

I'm building a diesel special soon, not VW but very similar in many respects, AX 1.4 idi diesel (excellent engine, bad alloy block).  I'm using a petrol GTI cast iron block and thinking about putting the later 1527cc head on the 1360cc block, which will drop compression to 20:!, via a larger pre combustion chamber.

I also am thinking about full ceramic treatment.  I have read a research paper which showed that fuel economy of 5% better was achieved by dropping compression down to 15:1, of course starting in the cold may be hard but I'm not too concerned about that especially not at 20:!.

Wanted to start a theory & pratical discussion on lowering compression ratio, enlargin the pre chamber and *possibly* reduce pumping frictional losses by increasing the bore of the entry hole to the pre chamber cup, anyone messed with this stuff?

Oh by the way 1st stage is to see what mpg I can get from this motor, it did 71mpg in stock form before, I have several improvements to make and a couple on the vehicle as well (narrower taller tyres, better fluids, aero engine guard, electric brake vacuum pump if I can find one).  2nd stage may be a turbo to see how quick I can make it, so a low compression and coated engine would be good prep for that.

I don't want to confuse with leaking rings/valves/worn out low compression, I'm talking a 100% fit and fettled engine, but working on bigger pre-combustion chamber size which is a different thing to an engine that won't start because all the compression is coming out the breather, obviously that's a big waste of power!

Greg.

Are you in the UK?

My father's AX does 86+ MPG imp on a run and abot 68mpg on the village farting about. It has a funny problem with the I/P where it surges like a turbo. [May have rcently stopped actually] Bosch or Citroen  don't know what it is but I suspect something stretched inside. Strangely a couple of months ago someone gave two AX's away because the better one had developed this same fault and failed the MOT and she didn't have the time to r & r ... Missed them  :cry:    Dad's is running a CAV setup  and these run with lower injection pressures than Bosch. I suspect that tuned up it will be more like a Renault 5 turbo Gordino (showing my age) and leave most Golfs behind.
IMO good injectors running less pressure are better mpg wise.

[greg123]

Yes I am in the UK.  The 71mpg was on a motorway run between 70 and 80 for around 400 miles, with a little running around.  I doubt 86mpg is obtained 2 up at 70+mph, are the figures obtained over several tank fulls by brimming the tank, zero trip counter, wait till empty, brim tank at same pump method?  Using this method over several thousand miles the best we had was 65mpg as an overall, at least 1/3 of that being urban.  I have never driven anything else that comes that close, the nearest being a Montego 2.0di turbo which would do 55mpg average but I could not better the average much on a run.

You are right about the engines, they do run very well and go very well.  However serious power would need a turbo as they are in the 54hp range for the 1.4 if memory serves correct and I calculated I will need 165hp to get a semi respectable 5 sec 0-60 time (grip allowing!).

I have had some surging during warm up issues on the CAV pump, other than that an unsuitability for veg oil, no issues.  I have a bosch pump for one sat here, not tried it as most of them (all the ones I have run) have been cav.  The 1.5 only came with cav setup.  I also have a brand new EPVE turbo pump from a pug 1.9, I should be able to fit this easily - plan to use it if I turbo the engine.  I have the injector lines and pulley to make it fit.  I might just use it anyway and see what happens, it will run fine as a na pump!  If the timing for the td 1.9 and the na 1.4 are similar I can't see an issue other than having to reduce max fueling a bit?

Question is, will I see gains in mpg from a 'worked' (ported and engine full ceramic treatment) 1.5 diesel head and the 20:1 compression ratio.

If lower compression wasn't better for efficiency, surely lorry engines would be running 25:1 rather than dropping them to scrape 15:1 at present?  A di will still work fine at higher compression ratio....

I guess the 1.5 head is for certain going to be better if I move away from the economy project to the 'power' project, if I build the engine with good studs and gasket and coated as a starting point it could evolve into either right?

Are you in the UK?

My father's AX does 86+ MPG imp on a run and abot 68mpg on the village farting about. It has a funny problem with the I/P where it surges like a turbo. [May have rcently stopped actually] Bosch or Citroen  don't know what it is but I suspect something stretched inside. Strangely a couple of months ago someone gave two AX's away because the better one had developed this same fault and failed the MOT and she didn't have the time to r & r ... Missed them  :cry:    Dad's is running a CAV setup  and these run with lower injection pressures than Bosch. I suspect that tuned up it will be more like a Renault 5 turbo Gordino (showing my age) and leave most Golfs behind.
IMO good injectors running less pressure are better mpg wise.

[greg123]

Just a comment, been thinking and maybe this is correct:

Providing there is enough heat and the air is swirling well in the pre-combustion chamber, allowing the diesel to ignite easily, further increases in compression will only result in increased losses in performance due to pumping and air friction losses - the extra heat energy gained won't improve matters since it was alreayd above diesel flashpoint.

So a rule of thumb all things being equal is, find the lowest compression ratio it will happily run at.  I also theorise that lowering cr by dishing the piston or similar would reduce in far less oxygen/air volume in the pre comp where the diesel is injected, meaning a bad burn.  So I would say that decrease in cr should only be done via a bigger pre comp volume, thus the amount of air in there is the same and indeed there is increased room for the mix to swirl before hitting the walls of the chamber, the advantage being lower pumping/friction losses.

Next question, why do mfr's produce high cr idi's if lower is better.  I think I may be able to suggest that cold starting (poorer with lower cr) cost (don't have to ceramic coat the engine etc) and reliability (when compression drops a bit a low cr engine may not run, but a high cr engine will still run - albeit breathing heavily and sounding tired - but will still manage another 50k!) so for mfr's a higher cr may be needed.

An enthusiast who has a good condition motor and can cope with/doesn't have to worry about freezing starts (there are ways round this anyhow, cold start fluid injection system/pre heater mods/block heater/a garage) may be a different kettle of fish right?

Comments please!

[saurkraut]

Read this too:

http://tinyurl.com/2nz2to

[jimfoo]

well gasoline might not be the greatest option, but i'm sure there is something out there (other than quickstart...)

i thought that the higher compression ratio requires more work to achieve, therefore lower compression ratio relieves that stress... so it's more efficient to have higher compression ratio's?? why do race gas engine's use higher CR's then???

Yes, higher CRs are more efficient, which is why race cars use them. More hp  and torque for the same displacement. However race cars have to use very high octane to prevent damage from the high CR. That fuel is much more expensive, and may still use lead to achieve those numbers. When I raced, the pump at the track said something like 4 grams, or maybe milligrams of lead per gallon, with 104 octane. It also cost $3.75/gallon back in 1994. That is one of the reasons diesels are more efficient, along with running lean, and the main reason they have more torque for the same displacement.

[QuickTD]

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.

 Fill a gasser with any heavier fraction (diesel/kerosene/jet A) by accident, even a small amount, and you will experience preignition that must be heard to be believed. Gasoline easily forms explosive mixtures with air but is not particularly succeptable to autoignition due to high temperatures. Be careful not to confuse flammabilty with octane (or cetane) rating, they are not interchangable terms. Diesel has very high tendency to autoignite, or low octane rating and cannot be used in a homogeneous charge (premixed fuel/air) gasoline engine as it will ignite long before the spark has a chance to fire it.

[Mark(The Miser)UK]

Is a higher compression ratio more efficient?  For any given engine, increasing compression ratio leads to higher chamber temperature. For the same head then heat transfer across to the coolant is greater. therefore less efficient. Definitely more power but I think not more efficient.
 Having said that the efficiency peak lies somewhere on a bell-curve; too much  or too little compression then it drops off. I suspect that higher  injection pressures help to hide defects in injectors a bit like strong valve springs give/maintain a good seat for the usual short life of the car.

Greg that mid 80's mpg is obtained doing between 60 and 70 mph cutting across from Gloucester to Ipswich via 50 roundabouts mum and dad fully loaded up for a weekend with my sister. A tank of fuel lasts so long that he doesn't normally check mileage. :lol:
Yea I mean't with a turbo added.  :wink:  Dad thinks there was a version with a turbo.
The other amazing thing is that he can put 15 bags of cement in the back and still pull a trailer with the same. This is one car that American dieselheads would have loved... Mind you my FIAT127 1300GT used to beat xr3i's Alas it was murderd by vandals.

[jimfoo]

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.

[greg123]

I am talking purely diesel here, not petrol - but to answer a few points below I disagree that higher the cr the more efficient, or you would see petrol race cars with 25:1 cr.  You don't, as they would detonate all over the shop and produce little or no power.  A drop from 25:1 to around 10:1 - 13:1 is required to achieve good power in a petrol engine.  Which shows there is an 'optimum' cr for a petrol, which allows for a quick ignition and fast burn resulting in peak pressure reached early in the cylinder cycle and more 'work done' on the piston. Higher octane petrol allows for earlier ignition and more pressure build up before the piston travels down, greater overall force on the piston.  Just to confuse matters not many people realise there is a BIG difference between static and DYNAMIC compression ratio, the former is what we talk about, the latter is the only thing that matters.  A cr is just an influence on pressure in the cylinder prior to ignition, a race petrol has massive valve overlap meaning that half the air leaks out of the valves at low speed resulting in low cylinder pressure even with a high CR - a low cr and it may not even run.  As revs rise and the valve timing means better cylinder fills the effective pressure gets better, but this is balanced by reduced cylinder filling due to speed, if memory serves at 6k roughly half the air per stroke is entering the cylinder due to the small amount of time allowed for the cylinder to fill and the inertia effects of the weight of the air etc, compread to low speed.  This of course is balanced by the fact that though a less efficient fill of the cylinder produces less power per stroke there are 4 times as many strokes - so overall power goes up.  As the cylinder pressures fall on a high rpm race engine the high cr helps to keep pre-ignition pressure up and get a fast burn going - needed to extract any useful power from the small amount of time the flame front has before the piston hits BDC.  Additionally petrol engines are throttled, with low throttle openings little air goes in and thus cylinder pressures are low, why some petrols like WOT to start better. LPPG is an interesting fuel as can tolerate 15:1 cr, or a far higher turbo boost and has other engine benifits.  Including charge cooling - if injected in liquid form.

Diesel has around 14% more energy per gallon than petrol, which is why partly it gets better mpg.  I belive the rest is down to the fact it can burn in an air rich environment (petrol requires a measured balanced environment - cannot burn as lean and requires to be vaporised which is hard to do 100%) so I still don't see high CR as anything other than an aid to getting the diesel to fire and swirl.  A tried and tested way of getting tired old diesels with no compression to start (witnessed this on a lorry engine in a digger recently) is to start a big fire of petrol soaked rag just below the intake, then crank the engine - it sucks in flames and red hot air from the fire and guess what - fires right up!  A blowlamp pointed into the intake of a small diesel has similar effect.  This goes to show that a diesel fires on heat + air, not necessarily compression - the compression is just a method of giving us enough air and the head in one place round the injector.

I have read the ceramic coating thread and much other research on the web and think I will go that direction anyhow which should help with air temps at point of ignition.

I'd like more discussion on the efficiency effects of lower CR if anyone has further to add?  I know I added about petrol systems above, but I guess that should mainly be for another thread as petrol is really a different animal to diesel in many ways, only the mechanical componants are similar (pistons, valves etc) the chemical bits of what's going on in the cylinder are very different.

At uni when doing automotive engineering I recall some single cylinder variable cr (rais and lower the block, crank stays in same place) engine that was built to study this, in my opinion it was flawed as it did not alter the cr via the swirl chamber capacity (or piston bowl capacity in a di) but I would like to dig up the research if I can.  The main thing I remember is that if it was low cr it wouldn't start - but we know that ;-)

Greg.

Diesels

Yes, higher CRs are more efficient, which is why race cars use them. More hp  and torque for the same displacement. However race cars have to use very high octane to prevent damage from the high CR. That fuel is much more expensive, and may still use lead to achieve those numbers. When I raced, the pump at the track said something like 4 grams, or maybe milligrams of lead per gallon, with 104 octane. It also cost $3.75/gallon back in 1994. That is one of the reasons diesels are more efficient, along with running lean, and the main reason they have more torque for the same displacement.