# From the Archives Archived discussions on CrazyEngineers

@zaveri • 06 Apr, 2012
if you check out the technical specifications of any vehicle and look under the engine section , there will be a parameter which will go like :

max.torque : x N.m @ y RPM

what exactly does that mean ? does it mean the maximum torque which the engine can generally provide or does it refer to the torque which is required to move the vehicle from rest ?

and similarly while designing a new vehicle and determining the torque required to push the vehicle from rest, how should we arrive at the engine speed(rpm) which would provide this starting torque ?
@mechky • 08 Apr, 2012 • 6 likes Answer to your first question-

It indicates the maximum torque the engine can provide and the rpm at which the engine is providing this maximum torque. Maximum torque of an engine is not at the highest rpm of the engine. The torque increases with increasing rpm, reaches its maximum value(at a certain rpm) and then begins to fall as the rpm further increases. Same is true with the power of the engine. There are many design & operational aspects of the engine which are responsible for this.

The torque curve depends on how well the cylinders are filled with fresh mixture, how well it burns that mixture and losses in that process. The maximum torque will be the point at which the engine is having maximum volumetric efficiency.After a certain rpm the filling ability of cylinders or the aspiration of engine falls down. Also the internal frictional losses also increase with increasing rpm. So, the net result is decreasing torque.​

1. The torque required to move a vehicle from rest is not equal to the torque which is at the flywheel of the engine. We have gearbox in the car by which the torque at the automobile wheels is more than the torque at engine's flywheel. When this torque produces enough force at the wheels, the car moves. So, when you determine the torque required to move a vehicle from rest, it is not the engine torque.

2. For arriving at the rpm of the engine at the moment when the vehicle moves from rest you need-
a) the torque vs rpm plot of the engine
b) the torque multiplication by the gearbox & differential
c) the minimum torque required to move vehicle from rest

By knowing (b) & (c), you can calculate the engine torque at flywheel at the moment when the vehicle starts. Then from (a) you can get to know the rpm.

I hope this helps.😀
@CE Designer • 10 Apr, 2012 mechky so what would be a good engine rating then, would it be maximum torque occuring at a low RPM or vici-versa?
In other words, if you have two sets of engine specs, each showing the max torque and rpm, which would you say is better in terms of speed and performance?
@ISHAN TOPRE • 10 Apr, 2012 • 1 like The best engine would be the one which gives max. Torque at low RPM. If you put it in layman's words, it means you are getting high power at less effort.
@mechky • 11 Apr, 2012 • 4 likes
CE Designer
mechky so what would be a good engine rating then, would it be maximum torque occuring at a low RPM or vici-versa?
In other words, if you have two sets of engine specs, each showing the max torque and rpm, which would you say is better in terms of speed and performance?

A better engine will be the one which gives max torque at relatively low rpm. Consider data of two engines-
Engine 1: Torque 99@2800, Power 67@5500
Engine 2: Torque 84@3500, Power 65@6200
For comparison, divide torque by rpm & power by rpm.
Engine 1: 99/2800=0.035, 67/5500=0.012
Engine 2: 84/3500=0.024, 65/6200=0.010

So, we can see Engine 1 is more powerful as it generates more torque at lesser rpm and more power at less rpm.

Torque is the deciding factor for a vehicle's traction i.e how much load it can pull. At low speed it is the torque which moves the car. Engine's power becomes dominant at high rpm. The vehicle's maximum speed depends on power.
@vikaskumar11233 • 13 Apr, 2012 I like this discussion.Hi guys can you please tell me what is hybrid engine?
@mechky • 14 Apr, 2012 • 2 likes
I like this discussion.Hi guys can you please tell me what is hybrid engine?
https://hybridreview.blogspot.in/2006/02/hybrid-engine.html
@zaveri • 18 Apr, 2012 • 1 like thanx mechky

this helped a lot
@ABCD ABCD • 26 Apr, 2012 Hey! how do vehicle with 2 or more engines work?
@zaveri • 26 Apr, 2012
mreccentric
Hey! how do vehicle with 2 or more engines work?
2 or more engines ? well honestly speaking, i have never heard of anything like that. and to the best of my belief i don't think any land vehicle has more than one engine.

if it is ever required that a vehicle needs more power, then all that is required to be done is to fix an engine that would give that much power, and there is no need to think of compounding two or more smaller engines.

Or maybe you have confused engine-cylinders with engines, then let me tell you that an engine can have upto 8 cylinders. (as in v-8 engines)
@ISHAN TOPRE • 26 Apr, 2012
mreccentric
Hey! how do vehicle with 2 or more engines work?
I think you are talking about Hybrids. They have two sources of power.

https://www.howstuffworks.com/hybrid-car.htm

zaveri
let me tell you that an engine can have upto 8 cylinders. (as in v-8 engines)

zaveri You are practically right but technically wrong. You can have as much power as you want. Have you heard about W16?
@zaveri • 26 Apr, 2012 @ issue

w 16 ? is that an engine with 16 cylinders ?

and as for hybrids, there is an ic engine along with an electric motor, but the two of them do not power the vehicle at the same time
@ISHAN TOPRE • 26 Apr, 2012 W16 is as you guessed correctly, a combination of two V8 engines.

Yeah, there is only one engine as far as I have heard. I was only asking mreccentric if he was talking about Hybrids.
@zaveri • 26 Apr, 2012
Issue
W16 is as you guessed correctly, a combination of two V8 engines.

Yeah, there is only one engine as far as I have heard. I was only asking mreccentric if he was talking about Hybrids.
if there are two v8 engines side by side, then does that mean it has two crankshafts ? !
@mechky • 26 Apr, 2012 • 2 likes
zaveri
@ issue

w 16 ? is that an engine with 16 cylinders ?

and as for hybrids, there is an ic engine along with an electric motor, but the two of them do not power the vehicle at the same time
W16 is a sixteen cylinder engine in W configuration. It has single crankshaft. The very famous Bugatti Veyron runs on W16 engine.

There are two types of hybrid technologies- series & parallel. In series hybrid, the gasoline engine starts the car and after attaining a certain level of speed the electric motor takes over automatically. In parallel hybrid both the gasoline engine and electric motor power the vehicle at the same time as and when required.
@zaveri • 27 Apr, 2012
mechky
W16 is a sixteen cylinder engine in W configuration. It has single crankshaft. The very famous Bugatti Veyron runs on W16 engine.

There are two types of hybrid technologies- series & parallel. In series hybrid, the gasoline engine starts the car and after attaining a certain level of speed the electric motor takes over automatically. In parallel hybrid both the gasoline engine and electric motor power the vehicle at the same time as and when required.
cool......i got it about the hybrids
@ABCD ABCD • 28 Apr, 2012 @ issue and @ zaveri
Yeah! The ques was for hybrid cars. I came to know that they both wont power at same time. But yet, i didn understant when they actually keep switching (slow speed = electric motor, high speed = IC engine). In parallel technology, how can they both run the vehicle at same time? I mean how a motor and engine can run the vehicle at same time.

Please also tell me how engine with 2 or more cylinders work.
@ISHAN TOPRE • 29 Apr, 2012 An engine work because of crankshaft. The cylinders are mounted on crankshaft. Now depending on position of cylinders, there are many times of engines.

Here is something that might interest you-
https://www.howstuffworks.com/engine.htm
@zaveri • 29 Apr, 2012 • 1 like
mreccentric
@ issue and @ zaveri

Please also tell me how engine with 2 or more cylinders work.
a single-cylinder engine has a very heavy flywheel, and the principle that governs flywheels are moment of inertia. when the piston does a power stroke, the flywheel is rotated through 180 degrees and then it does not stop easily due to its heavy mass, and its rotation keeps the crankshaft in rotation, which in turn makes the piston to do the exhaust, suction and compression strokes.

after this a fresh power stroke takes place and the cycle continues.

in a 4-cylinder engine, each piston in a cylinder does a different stroke of the cycle, which means that no 2 pistons does a same kind of stroke at the same time, and thus a power stroke is obtained at any given time.
@ABCD ABCD • 04 May, 2012 a silly question, how is rotary force from engine sent to wheels. Is it front engine/back engine(for cars)?
@mechky • 04 May, 2012 • 2 likes
mreccentric
a silly question, how is rotary force from engine sent to wheels. Is it front engine/back engine(for cars)?
It is not at all a 'silly question'. You won't believe it but there are many mechanical engineers out there who have no idea about this, apart from those working in auto industry.
There are three types of drive systems in automobiles - Front wheel drive, Rear wheel drive and Four wheel drive. Most of the small and mid-segment cars in India are using front-wheel drive. So, I will explain the front wheel drive.
In case of front wheel drive, the engine drives the front wheels only. We have usually the transverse layout of engine in which the crankshaft is connected to transmission through clutch & transmission is further connected to differential. The differential finally drives the two axles of the two front wheels.

The image below gives a simple explanation of how things work-

This image shows how an exact front wheel drive system with suspension and steering system looks like(differential is not shown and is actually located between the driveshafts)-

@ISHAN TOPRE • 04 May, 2012 An engine may be located in front or back. The power is transmitted to the wheels through a differential mechanism.
@ABCD ABCD • 04 May, 2012 I had heard that differential is used only for steering the vehicle.
@zaveri • 05 May, 2012 • 1 like
mreccentric
I had heard that differential is used only for steering the vehicle.
The differential is a device which only enables a smooth operation during turning. It however does not directly steer the car.

the steering is carried out by the movement of the front wheels . the differential is however not connected to the steering wheel or steering gear kinematically, but it starts functioning when dynamic forces act on the car when it negotiates a turn.

the function of the differential is to prevent tire wear of the rear wheels, and to prevent the engine from stalling when the car takes a turn.

here is something that will interest you :

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