Go Back   Team-BHP > Under the Hood > Technical Stuff


Reply
 
Thread Tools Search this Thread
Old 26th April 2010, 10:19   #31
Distinguished - BHPian
 
sgiitk's Avatar
 
Join Date: Dec 2007
Location: Kanpur
Posts: 7,160
Thanked: 3,755 Times
Default

There are two reasons.

1. Diesels are not high revving, hence oversquare has no real advantage.
2. With the high compressions of diesels an oversquare combustion chamber will be tough to design. May even have been impossible in an indirect injection scenario.

Last edited by sgiitk : 26th April 2010 at 10:22.
sgiitk is offline   Reply With Quote
Old 26th April 2010, 19:26   #32
Senior - BHPian
 
oxyzen's Avatar
 
Join Date: Feb 2010
Location: Not fixed.
Posts: 1,026
Thanked: 265 Times
Default

Quote:
Originally Posted by Mpower View Post
Are they? can you name a few?

The only oversquare ones are high revving perf engines like BMW M5, ferrari.

My bad.
I meant undersquare.
Extremely sorry.
oxyzen is offline   Reply With Quote
Old 3rd May 2010, 11:18   #33
BHPian
 
handsofsteel's Avatar
 
Join Date: Jun 2009
Location: mumbai
Posts: 200
Thanked: 133 Times
Default

Quote:
Originally Posted by oxyzen View Post
Mostly all engines are oversquare (petrol/ diesel)
Diesels have comparatively more stroke length. Here I will explain why petrols have shoter bore length.
The main objective of using an overquare engine is th maximise the cyl head area. More is the cylinder head area more is the amount of valve area you can accomodate.Practically speaking torque depends on the air intake per stroke. And power deprnds on the air intake per second. Intake valves are the biggest restriction for the intake air passage. Smaller valve area will be ok for low RPMs but at higher RPMs you need more valve area which means more cylinder head area, ie oversquare engines. However the diggest drawback of oversquare engine is that the flamefront has to travel a longer distance, so the rate is combustion is effectively slower. Leading the engine prone to knocking.

Smaller but significant points.
---Longer stroke brings down the exhaust temperature. Prevents cata from melting.
---Shorter stroke means less frictional losses but more blowby production.
---Turbulance (Swirl,Squish, Tumble) are more related to the chamber design than oversquare/undersquare.
the CI engine has no concept of flame front or flame travel as t
he combustion occurs in several places at once. your reasoning is relevant only for SI engines.
handsofsteel is offline   Reply With Quote
Old 3rd May 2010, 11:24   #34
BHPian
 
handsofsteel's Avatar
 
Join Date: Jun 2009
Location: mumbai
Posts: 200
Thanked: 133 Times
Default

Quote:
Originally Posted by oxyzen View Post


---Shorter stroke means less frictional losses but more blowby production.
again, i think this is incorrect, because, oversquare pistons have higher rpm inherently and therefore more stokes per minute increasing the rubbing against the wall. also, friction increases with speed therefore the friction losses should increase.
handsofsteel is offline   Reply With Quote
Old 3rd May 2010, 13:20   #35
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Quote:
again, i think this is incorrect, because, oversquare pistons have higher rpm inherently and therefore more stokes per minute increasing the rubbing against the wall. also, friction increases with speed therefore the friction losses should increase.
This need not be true in all cases.

If you take Honda's K20 series. All engines have 86mm x 86mm but their power and torque rpms vary hugely.

The K series found on the Civic Type R produces max power@8000rpm and max torque@6100rpm, while the K series found on the Civic Si does max power@6500rpm and max torque@4000rpm. Both engines have the exact same bore, stroke and engine capacity.

Now these were NA engines.

But on a turbo diesel, you could have a short stroke engine designed to produce power and torque at low rpms, with enough boost to produce healthy power/torque.

So this lets you achieve high torque at lower revvs while making sure the pistons aren't getting worked too hard when compared to a long stroke engine at the same rpm.

And i think a short stroke design also reduces the stress on the crankshaft (due to lower piston speed vs engine speed).

Shan2nu

Last edited by Shan2nu : 3rd May 2010 at 13:22.
Shan2nu is offline   Reply With Quote
Old 3rd May 2010, 13:50   #36
BHPian
 
handsofsteel's Avatar
 
Join Date: Jun 2009
Location: mumbai
Posts: 200
Thanked: 133 Times
Default

Quote:
Originally Posted by Shan2nu View Post

If you take Honda's K20 series. All engines have 86mm x 86mm but their power and torque rpms vary hugely.

The K series found on the Civic Type R produces max power@8000rpm and max torque@6100rpm, while the K series found on the Civic Si does max power@6500rpm and max torque@4000rpm. Both engines have the exact same bore, stroke and engine capacity.
the engine output depends on a variety of factors like combustion chamber
geometry (piston shapes, valve layout),in-cylinder charge motion, turbulence generated, manifold orientation, injection timing, quantity, the shape of the injection pulse etc. to co-relate this with the FP (frictional power) alone would be incorrect.therefore, just because an engine produces more power at a lesser rpm than the other doesn't mean that
FP reduces with speed. (BTW, i genuinely appreciate the research undertaken to dig out that bit of data on the civic engines). in fact FP is directly proportional to the square of velocity. the point i had made was that for 2 identical engines, an engine at a higher rpm is incurring more friction losses.

Quote:
Originally Posted by Shan2nu View Post
And i think a short stroke design also reduces the stress on the crankshaft (due to lower piston speed vs engine speed).

Shan2nu
I don't know about that, because the mean piston speeds of most of the engines (automobiles) fall in the same band. even if such a thing were to exist, it would surely be marginal considering advances in the design these days. also, the flywheel would surely have been designed with this objective in mind.
handsofsteel is offline   Reply With Quote
Old 3rd May 2010, 14:44   #37
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Quote:
the engine output depends on a variety of factors like combustion chamber
geometry (piston shapes, valve layout),in-cylinder charge motion, turbulence generated, manifold orientation, injection timing, quantity, the shape of the injection pulse etc. to co-relate this with the FP (frictional power) alone would be incorrect.therefore, just because an engine produces more power at a lesser rpm than the other doesn't mean that
FP reduces with speed. (BTW, i genuinely appreciate the research undertaken to dig out that bit of data on the civic engines). in fact FP is directly proportional to the square of velocity. the point i had made was that for 2 identical engines, an engine at a higher rpm is incurring more friction losses.
And thats what im saying. Just bcoz someone has designed an oversquare engine, it does not mean they want to produce power/torque at higher revvs.

Honda used a 2ltr 86 x 86 engine to produce peak power at 8000 as well as at 6500. So the size of the bore and stroke don't always dictate where the power and torque are to be produced (which you have explained in your above post).

You can make a short stroke engine produce power at low revvs also.

The real reason why Mahindra has gone in for an oversquare design, only they can explain. But i have a hunch they had "friction" and "stress" on their minds (which i've already mentioned in post # 8).

Shan2nu
Shan2nu is offline   Reply With Quote
Old 3rd May 2010, 17:41   #38
Distinguished - BHPian
 
Mpower's Avatar
 
Join Date: Mar 2005
Location: Bangalore
Posts: 10,432
Thanked: 1,682 Times
Default

Quote:
Originally Posted by handsofsteel View Post
again, i think this is incorrect, because, oversquare pistons have higher rpm inherently and therefore more stokes per minute increasing the rubbing against the wall. also, friction increases with speed therefore the friction losses should increase.
Oxyzen is right. Oversquare config = lower friction losses. Also more blowby and more HC emissions due to the higher land area.
Mpower is offline   Reply With Quote
Old 3rd May 2010, 19:13   #39
Senior - BHPian
 
oxyzen's Avatar
 
Join Date: Feb 2010
Location: Not fixed.
Posts: 1,026
Thanked: 265 Times
Default

To decide the max torque (low RPM/ High RPM) the major component is Intake manifold and also valve configuration. More is the runner length lower is the RPM where u get max torque.
Another superficial example is of I10 irde . 1.1 irde has 2 valves per cylinder and it produces max torque at 2800RPM, while other models with 4 valves per cylinder generally have their max torque at 4000RPM. I think it is applicable for diesel as well.

In case of oversquare engines it is about sustainability at high RPM and not regarding getting max torque over there.

Quote:
Originally Posted by handsofsteel View Post
the CI engine has no concept of flame front or flame travel as t
he combustion occurs in several places at once. your reasoning is relevant only for SI engines.
Ya you are right.

Quote:
Originally Posted by handsofsteel View Post
again, i think this is incorrect, because, oversquare pistons have higher rpm inherently and therefore more stokes per minute increasing the rubbing against the wall. also, friction increases with speed therefore the friction losses should increase.
When you are considering 2 engines of the same displacement you cant compare one running at 6000RPM and the other running at 10000RPM. Its biased.
Hypothetically speaking an oversquare engine running at 10000RPM will have lower fr losses than an undersquare running at 10000RPM.(Considering both have same displacement and torque curve.)
oxyzen is offline   Reply With Quote
Old 3rd May 2010, 19:18   #40
Senior - BHPian
 
oxyzen's Avatar
 
Join Date: Feb 2010
Location: Not fixed.
Posts: 1,026
Thanked: 265 Times
Default

Quote:
Originally Posted by Skindleberg View Post
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)

Well I think the original question is answered.
Quote:
Originally Posted by handsofsteel View Post
the CI engine has no concept of flame front or flame travel as the combustion occurs in several places at once.
When its diesel we dont talk about flame.
How did i miss it.
oxyzen is offline   Reply With Quote
Old 3rd May 2010, 21:28   #41
Distinguished - BHPian
 
Mpower's Avatar
 
Join Date: Mar 2005
Location: Bangalore
Posts: 10,432
Thanked: 1,682 Times
Default

Quote:
Originally Posted by oxyzen View Post
Another superficial example is of I10 irde . 1.1 irde has 2 valves per cylinder and it produces max torque at 2800RPM, while other models with 4 valves per cylinder generally have their max torque at 4000RPM. I think it is applicable for diesel as well
IIRC, the 1.1 is 3 valves per cyl but the torque curve is v similar to a 2v engine.
Mpower is offline   Reply With Quote
Old 4th May 2010, 22:35   #42
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,506
Thanked: 2,066 Times
Default

Quote:
Originally Posted by oxyzen View Post
Well I think the original question is answered.
Any insight on the supplementary question?

Regards
Sutripta
Sutripta is offline   Reply With Quote
Old 5th May 2010, 16:07   #43
BHPian
 
jalsa777's Avatar
 
Join Date: May 2006
Location: Mumbai
Posts: 637
Thanked: 1,044 Times
Default

Quote:
Originally Posted by Sutripta View Post
Any insight on the supplementary question?

Regards
Sutripta
your answer lies in the ignition process of both engines.

Petrol:

A petrol engine is an air controlled engine. i.e.: The amount of air going into the engine is modulated to control the engine.

Air and fuel injected together during the intake stroke

Lets start from the compression cycle:
The fuel-air mixture compresses, this results in very high pressure and also high temperatures in the combustion chamber.
But, this is not enough temperature n pressure to self ignite petrol.

So, a spark is introduced to ignite the mixture.
There is usually only 1 spark plug, and it is located in the center at the top of the combustion chamber.

When this spark occurs, the fuel in the immediate vicinity of the electrode ignites.
These fuel particles cause the other fuel particles around them to ignite and so on.

Thus, the fuel ignites in a predictable manner across the combustion chamber. The flame propagates like light being emitted from a point source, in a spherical manner.

The boundary between the ignited fuel particles and the unignited fuel particles is called flame front.

this video will help you understand:



Also, the timing of the spark can be precisely controlled.
This is needed because, the rate at which the fuel ignites is an inherent property of the fuel.
It varies in a predictable manner with temperature, pressure and also depends on the quantity of fuel present in the cylinder.

All these factors contribute to make gasoline fuel ignition an almost exact science.
Thus, the ignition can be precisely initiated and also its termination time is known.

This allows engineers to push the limits of speed...resulting in high RPMs.



Diesel Engine:

It is a fuel controlled engine. The amount of air going in is not controlled but the fuel injection time is controlled.

In the compression cycle, only air is present and it gets compressed to attain much higher temp. and pressure than petrol engine.
When the conditions are right for the fuel to be self-ignited, the fuel is then injected at the end of the compression stroke.

This fuel takes some time (in microseconds) to ignite, during this time the fuel spreads throughout the combustion space.

Then the fuel ignites, but there's a catch.
From where will it ignite?
Will it all ignite together?

There are the questions to be answered which makes it unpredictable.

Each fuel particle behaves like a spark plug.
The time when it will ignite depends upon:
  • Local temp. and pressure around it
  • The time it has spent in these conditions
  • Other fuel particles around it

All this cannot be predetermined.
It can be controlled to an extent with the help of design (piston head shape, swirl, nozzle properties, valve position, etc)
but can not be controlled.

This is what makes diesel combustion unpredictable.

I hope this satisfies your question

I have made it as simple and short as possible.
There are a lot of more factors like, engine temperature, valve head temps., chamber design, additives to fuel, fuel properties, jacket temp., air properties, etc. etc.

You can get more detailed info in any self-respecting book dealing with fundamentals of IC engines
jalsa777 is offline   Reply With Quote
Old 5th May 2010, 16:54   #44
BHPian
 
Join Date: Dec 2008
Location: Planet E.A.R.T.H.
Posts: 140
Thanked: 7 Times
Default

Quote:
Originally Posted by Shan2nu View Post
The real reason why Mahindra has gone in for an oversquare design, only they can explain. But i have a hunch they had "friction" and "stress" on their minds (which i've already mentioned in post # 8).

Shan2nu

The reason M&M have gone in for an 'over-square' design is because they already have a design (Peugeot xd3p) to which they will only be making cosmetic changes.

'Why fix it when it ain't broke?'

The new NEF 2.5 CRDe engine has identical bore,stroke and hence capacity as the xd3p. What has remained the same and what has changed? Only time will tell.
'72 Bullet is offline   Reply With Quote
Old 5th May 2010, 19:28   #45
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Quote:
The reason M&M have gone in for an 'over-square' design is because they already have a design (Peugeot xd3p) to which they will only be making cosmetic changes.
Can you post the technical details of this engine like cc, bore, stroke, comp ratio, peak power rpm, peak torque rpm.

Shan2nu
Shan2nu is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Truck Diesels vs. Car Diesels smashnerd Technical Stuff 7 15th January 2011 16:41
Diesels? Hybrids? Another 10 years max GTO Technical Stuff 8 8th March 2005 18:48
Any info on modifying diesels mmmjgm Modifications & Accessories 2 20th December 2004 15:44
Small Diesels. Which one? anshu12420 Hatchbacks 21 20th August 2004 17:51
Maruti-Suzuki now dedicated to Diesels!! GTO The Indian Car Scene 3 10th April 2004 15:14


All times are GMT +5.5. The time now is 23:59.

Copyright 2000 - 2017, Team-BHP.com
Proudly powered by E2E Networks