[Skindleberg]

Hi everyone

Does anyone know why diesel engines are mostly undersquare? There have been engines like the cummins val, vine, vim and the peugeot xd3 which was on the bolero and armada grand. But it seems to be a rare moment in history when a company has launched an overaquare diesel.
Anyone know why?
It is certainly not because '...diesels need high compression ratio...blah blah' because you can get high compression ratios on oversquare stuff too.

I read this bit of literature on subaru's boxer diesel (it being a square engine 86x86) website that oversquare designs cause problems in the stability of combustion in compressed ignition engines. Thats about all they said...one sentence!
Anyone know why is the flame unstable or any other serious disadvantage.

(No sparky '...it wouldn't be fun that way' does not qualify as a serious disadvantage)

[scuderiamania]

I have no idea as to what an oversquare diesel engine or an undersquare diesel engine is all about. Can some please elaborate?

[Manoj2268]

^^
Oversquare engines are those engines in which the stroke(length of the cylinder) is shorter than the piston diameter. In an undersquare engine, the stroke length is longer than the bore.

[dinar]

with new generation fuel injection system, one does not need over square diesels for getting power on of them, FIAT's multi jet.The power band is an issues with square and over square diesels as with all diesels, but added disadvantage is due to the short stroke of the engine it can neither get full advantage of torque nor can it stay in bhp band.

[DRIV3R]

@Scuderiamania
Oversquare - Stroke length is shorter than the bore dia, otherwise called short stroke engines

Undersquare - The vice versa

[Sutripta]

Originally Posted by Skindleberg

Hi everyone

Does anyone know why diesel engines are mostly undersquare? There have been engines like the cummins val, vine, vim and the peugeot xd3 which was on the bolero and armada grand. But it seems to be a rare moment in history when a company has launched an overaquare diesel.
Anyone know why?
It is certainly not because '...diesels need high compression ratio...blah blah' because you can get high compression ratios on oversquare stuff too.

I read this bit of literature on subaru's boxer diesel (it being a square engine 86x86) website that oversquare designs cause problems in the stability of combustion in compressed ignition engines. Thats about all they said...one sentence!
Anyone know why is the flame unstable or any other serious disadvantage.

(No sparky '...it wouldn't be fun that way' does not qualify as a serious disadvantage)

For a given capacity, different architectures (No. and configuration of cyl, bore/ stroke etc) have different characteristics. Whether these are advantages or disadvantages depend on what you want, and your perspective.

For the same mechanical stresses, short stroke engines can rev more. Also the larger valve area help breathing at the higher rpms. Together, more power! But in a diesel, rpm is limited more by injection/ combustion process. (Short time duration, relatively longer distances for the spray to traverse) So the higher rpm limit really cannot be utilised. Also the modern diesel is TC, largely masking the disadvantage of smaller valves. In a diesel, there is nothing to be gained by going short stroke. And you loose its famed lugging ability.

I believe some of the problems can/ have been overcome in the CRDIs. Experts comment please.

It is interesting that I have had experience of both the XDP 4.9 and the XD3. Good though they are (for a non turbo) never felt there was anything special compared to contemporary Japanese mills.
Have also used a Cummins VT. These might have been gods answer for trucks, dozers and earthmovers, but for normal stationary applications give me a NT or KT anyday.

I think you'll get better answers if you post your query in the 4x4 section. Mahindra engineers are active on that forum, and other than the XDs, their NEF is also, I believe, oversquare.

Regards
Sutripta

[Mpower]

Originally Posted by Skindleberg

Hi everyone

Does anyone know why diesel engines are mostly undersquare?

There have been engines like the cummins val, vine, vim and the peugeot xd3 which was on the bolero and armada grand.

Diesels? Almost all passenger car vehicles are undersquare. Only performance motorcycles are oversquare.

your second sentense makes no sense to me. Vim, Rin, surf ??

[Shan2nu]

Engine output wise, it maynot have much of an advantage on a turbo diesel but, maybe it was done keeping stress and friction in mind.

Shan2nu

[Sutripta]

Originally Posted by Mpower

your second sentense makes no sense to me. Vim, Rin, surf ??

Cummin's legendary V/ VT series.

Originally Posted by Shan2nu

Engine output wise, it maynot have much of an advantage on a turbo diesel but, maybe it was done keeping stress and friction in mind.

Shan2nu

Could not get what "it" refers to.

Regards
Sutripta

[Shan2nu]

Could not get what "it" refers to.

Over square pistons.

Shan2nu

[Skindleberg]

@Sutripta - I agree with your points and that is what I also understood from reading up here and there but I was hoping someone would have some info about generating turbulence inside the combustion chamber.

You've had the xd3 engine? how did it perform? anything vitally different from undersquare engines? there was also a turbo version of the xd3 I believe.

Do you also have any experiences with injector fouling?

Thanks

[Sutripta]

Originally Posted by Skindleberg

@Sutripta - I agree with your points and that is what I also understood from reading up here and there but I was hoping someone would have some info about generating turbulence inside the combustion chamber.

You've had the xd3 engine? how did it perform? anything vitally different from undersquare engines? there was also a turbo version of the xd3 I believe.

Do you also have any experiences with injector fouling?

Thanks

I think its time for the experts to step in and explain things like swirl, turbulence, squish, flame fronts etc, and the difference in the combustion process between petrols and diesels. And I believe with CRDIs, the fundamental cycle is shifting from isobaric to isochoric. A whole new ballgame.

Maybe the moderators can repost in the 4x4 section, where I think you'll get a better response.

XD3: No problems with injectors. When the XDP came out, compared to other engines then available in India (P3/4, Matador, KOEL RET3/ 4 etc) and esp. compared to the B275, it was a screamer. But compared to the Jap engines which were easily available here a few years later (2nd hand engines brought in as scrap/ spares etc), nothing special. Pls. note we are talking of a time which held DIs to be low revving limited powerband engines. One of the reason why IDIs were used in cars.

I believe Mahindra TCed the XD3 and sold it in SA. With unhappy results.

Regards
Sutripta

[SPIKE ARRESTOR]

Originally Posted by Sutripta

I think you'll get better answers if you post your query in the 4x4 section. Mahindra engineers are active on that forum, and other than the XDs, their NEF is also, I believe, oversquare.

Oversquare engines (XD3P, XDP4.90, NEF 2.49), Undersquare (MDI 3200, m-Hawk) Square (NEF 2.6).

Originally Posted by Sutripta

I think its time for the experts to step in and explain things like swirl, turbulence, squish, flame fronts etc, and the difference in the combustion process between petrols and diesels.
I believe Mahindra TCed the XD3 and sold it in SA. With unhappy results.

As told by sutripta the major criteria here is the combustion/flame characteristics of the IC engine. This brings into picture squish, turbulence, scavenging etc. With CRDi engine and various injection strategies (single pilot, double pilot, late pilot, early pilot) the "ignition delay" which resulted in gradual increase in peak combustion pressures has improved drastically resulting in better/flatter torque curves.

Spike

P.S. Sutripta have you driven the XD3P with 4.55 ratio? It luggs beautifully.

[The Wolf]

Going oversquare means producing torque at relatively higher rpms, which is against a diesel engine application...fox example- lugging heavy load or torquing through steep gradients on a higher gear. Plus this would mean loss in fuel efficiency due to useage of lower gears for longer to stay in peak torque band. It is not impossible to make a oversquare's produce power low down but that would mean complicating a simple thing deliberately by defying tested science to achieve undesirable results at higher costs...which business owner wants that?

In other words, diesel engines have a specific purpose, i.e lugging capability at lower rpms with comparitively lesser stress in mechanical parts involved in the equation hence making them that bit more reliable when it comes to broken parts due to mechanical stress.

To put the above in scientific terms, a diesel flame front unlike gasoline does not travel as fast and predicatably due to its inherent density and viscosity characterstics hence making a delayed and short flame entry exit making combustion a challenge...hence the useage of higher compression ratio (already adding to mechanical stress to internals and fuel lines due to higher pressure application). Under these conditions a oversquare will only add more stress on compnents making the over all build more susceptible and unreliable...plus losing its forte of high torque at low rpms throwing it out of contention in that segment (trucks, ships, earth moving equipments etc)

Hope that helps...

[Sutripta]

Forgot to mention about the throttle response of the XD3. Its pathetic. Tragedy is I feel it is not power related, but governing related. Most probably for smoke emission reasons, the governer is VERY gentle with the amount of fuel injected. There seems to be no difference between flooring the accelerator, and gently opening it up, in baby steps. Maybe the Mahindra engineers can confirm/ refute.

Sutripta