Download Oversquare engines - Team-BHP

Stroke
Bore
Crank Shaft Radius
Point 1:
Oversquare – Stroke > Bore or So < Bo
Undersquare – Stroke < Bore or Su < Bu
For a equal displacement cylinders, Oversquare will have less Stroke So < Su and more Bore Bo > Bu
Displacement = So x π (Bo/2)2 = Su x π (Bu/2)2
Point 2:
This implies So/ Su = (Bu/ Bo)2
Now coming to Power and Torque calculations,
Assuming same compression ration; Pressure inside the cylinder would be the same for both.
Force exerted by Cylinder = Pressure inside Cylinder X Area of the Piston
Fo = P x π (Bo/2)2 and Fu = P x π (Bu/2)2
Fo > Fu since Bo > Bu
Point 3:
Fo > Fu
Now this Force is converted to torque at the crank shaft radius
Torque produced = Force x Radius of Crankshaft (simplified for understanding)
To = Fo x Rco and Fu = Fu x Ruo
Now Rco = So/2 and Ruo = Su/2 so
Rco < Ruo
So To = Fo x So/2 and Fu = Fu x Su/2
Also To = P x π (Bo/2)2 x So/2 and Tu = P x π (Bu/2)2 x Su/2
Point 4:
To = Tu from Point 2:
We can see that 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 Power = 2 π N T / 60 where N is in RPM of the shaft
Point 5:
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.