[Mi10]

From a layman point of view:

Most oversquare engine produces peak torque and power at higher RPMs whereas its exactly the opposite in undersquare engines.

Now you should note that there is a directly proportional relationship between engine life and higher revving.

Most passenger cars (both petrol and diesel) use undersquare engine because of the following:

• The engine should do a bare minimum of atleast 2 lac kms, giving an oversquare engine will shorten this threshold
• Not everyone would like to revv the nuts of each time before a gear shift. Oversquare engine always tend to produce more vibrations than the undersquare / square counterparts.
• Oversquare engines give comparatively lesser miles per gallon when compared with like to like square / under square engines. This is because for getting into the power band you always tend to use WOT, which is not the case with square/undersquare engines.
• Now coming to diesels, when when comparing undersquare / square petrols to the petrols tend to be little more revv friendly than the diesels. All NA diesels produce torque at lesser rpms.

[ramzsys]

The premier padmini came up with a diesel engine, borrowed from FNM Italy who actually was a marine diesel engine maker. The bore stroke was 78x71.5. This design is to increase the long term high rpm reliability i suppose, as required for marine boats.

[Sutripta]

Originally Posted by SPIKE ARRESTOR

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

Hi Spike,

Lets leave gearing out of it!

BTW, have you seen my Armada with the Nano engine which is permanently stuck in low range (damn the electric shift). It is a beautiful lugger

Sutripta

P.S. Have started a thread. Comments welcome!

[thamban]

The full write-up is attached as a Word Document.



Oversquare engine Torque is theoretically equal to Undersquare engine Torque but practically Oversquare engines are designed so that it has a stroke above minimum So for mechanical reliability. We practically see that most high revving engines typically have low low-end torque.


Now for the same RPM, N the Oversquare cylinder piston has to move less distance up and down than the Undersquare cylinder piston (So < Su). This improves the high revving performance of Oversquare engines since it easier for them to do the same RPM compared to an Undersquare engine of same displacement.

So a high-revving engine is desired to have Oversquare engines. Performance car engines and bikes need high speed capabilities that translate to high RPM because of which they are made Oversquare.

The characteristics of these Petrol engines are that it has almost flat torque curve and so a linearly increasing Power Delivery Curve with RPM. So the driving method is to drive at higher RPMs to get the most out of the machine.

Now coming to Diesel engines, the important characteristics of a NA Diesel engine is its high Torque at lower RPM. This makes it desired for applications having high inertial load, for e.g trucks, heavy machinery etc. These are not high speed application but applications with higher initial load.
This also means that NA Diesel engine has a non linear power delivery which means the engine will deliver less power at higher RPMs than at lower RPMs

Consider a Trailer climbing a hill. It needs lots of Torque at lower speeds. Now if we were to fit a petrol engine to the Trailer it has to be always kept near to the red line and also at lower gears so that we get the maximum power out of the engine. This leads to higher fuel consumption and engine stress. If the Trailer is having a NA diesel engine then it can climb at lower rpm at a higher gear, which translates to higher fuel efficiency and less engine stress.

Now if we were to fit a Diesel engine into a performance car, then we need a linear power delivery. This is where the turbo-charged engines come into picture. When we use Turbo charged Diesel engines we are actually increasing the Power delivery at higher RPMs. But inherently Diesel engines cant high revv as much as petrol engines because of uncontrolled ignition which is somewhat eliminated by direct injection, but still it lags behind Petrol engines in high revving capability.

So an Oversquare Diesel engine is to be used when there is a need of high speed operation. The Petrol engines outclass Diesel engines in high speed operational performance any day. So there is not much incentive to produce Oversquare Diesel engines except for maybe high speed boats or tugs or where you need high speed performance and huge initial load bearing capability.

[SPIKE ARRESTOR]

Originally Posted by Sutripta

Hi Spike,

Lets leave gearing out of it!

BTW, have you seen my Armada with the Nano engine which is permanently stuck in low range (damn the electric shift). It is a beautiful lugger

Sutripta

P.S. Have started a thread. Comments welcome!

Armada with Nano engine? Didn't get you, Will go through the other thread.

Spike

[Sutripta]

Originally Posted by SPIKE ARRESTOR

Armada with Nano engine? Didn't get you, Will go through the other thread.

Spike

Just joking man. Trying to get across that with proper gearing anything will lugg.

Originally Posted by thamban

....
But inherently Diesel engines cant high revv as much as petrol engines because of uncontrolled ignition which is somewhat eliminated by direct injection, but still it lags behind Petrol engines in high revving capability.
....

What is that? And why? This was the original question, and a definitive answer is still awaited. I doubt the " somewhat eliminated by direct injection" bit because pre CRDI/TDI days, IDIs revved better than DIs. So it has to be the injection method (CR/ PD) rather than the DI. Am not upto speed on CRDIs.

Have started another thread on the characteristics of long and short stroke engines. Comments welcome.

Regards
Sutripta




There is

[oxyzen]

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.

[conjon]

Originally Posted by oxyzen

---Turbulance (Swirl,Squish, Tumble) are more related to the chamber design than oversquare/undersquare.

isn't it the design of piston crown that aids the turbulence?

[oxyzen]

Originally Posted by conjon

isn't it the design of piston crown that aids the turbulence?

Yes you are right. Turbulence depends mainly on the design of piston crown design. There are other factors also like the
Intake port area. (Smaller the area higher is the turbulence.)
Intake port angle / orientation.
Any obstruction. (Some times the spark plug is placed in the path of the air flow. It results in turbulence as well as spark plug cooling)
Turbocharging.

[Sutripta]

Originally Posted by Mpower

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

Originally Posted by oxyzen

Mostly all engines are oversquare (petrol/ diesel)

Members of the jury, have you reached your verdict?

Actually we are repeating the same thing over and over again. But the thread starters original question (Is there a problem/ instability in the combustion process in a big bore/ oversquare diesel, and if so what? ) is not being answered. Strange.

Regards
Sutripta

[Mpower]

Originally Posted by Sutripta

Members of the jury, have you reached your verdict?

You can do the research yourself on b x s modern engines. I have no idea about dinosaurs like XD3P

For diesels, you generally dont want cylinder sizes less than 350cc.

Originally Posted by oxyzen

Mostly all engines are oversquare (petrol/ diesel)
Diesels have comparatively more stroke length. Here I will explain why petrols have shoter bore length.

Are they? can you name a few?

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

[Sutripta]

Originally Posted by Mpower

You can do the research yourself on b x s modern engines.

Yes. I know. But what I know can always be wrong

As to why it is so can be the start of another thread. Efficiency maybe?

Regards
Sutripta

[greatmana2000]

many toyota diesel engines have a shorter stroke compared to the bore .

Actually over square /undersquare /flat engine design depends on the application . In diesel there is no spark ignition , so the air inside the cylinder must get hot enough to get ignited . So having a longer stroke helps the air inside the bore to get heated to the optimum combustion temperature .
Diesel engines are inherently low speed , high compression the torque due to the high compression ratio is used to advantage . The most important design aspect of an engine is the mean piston speed .

so in theorey it is bore /stroke/the biggest possible valve area for the engine in design and the mean piston speed should not be exceeded . else the engine will wear off and might break .

[1self]

Originally Posted by Mpower

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

Sonata 2.7 V6 is an oversquare engine. Bore is 86.7mm, stroke is 75mm.

[Mpower]

Originally Posted by 1self

Sonata 2.7 V6 is an oversquare engine. Bore is 86.7mm, stroke is 75mm.

When OEMs take an existing engine and create a new one with either more or less displacement, you do end up with oddball configurations like this.

For ex, take Toyota's new Valvematic range 1zr, 2zr and 3zr. All use the same 80.5 bore size which makes the head common the related CFD work common, makes the block more mass efficient and simplifies the block machining because the bore-bore spacing is identical. Stroke goes from oversquare on the 1.6 to waay-undersqure on the 2.0 version.