Engine Torque Definition and Formula:
What is Engine Torque?
Torque, in simple terms, is ‘Twisting or Turning Force’. It is the tendency of a force to rotate an object about an axis. In automotive terms, it is the measure of rotational effort applied on engine crankshaft by the piston.
Torque= Force x Distance. The SI system uses Newton-metre (Nm) to measure the torque. The other units are kilogram-meter (Kg-m) in metric and feet-pound-force’ (ft-lb) in imperial / British units.
Every engine is designed and built for a specific purpose. Hence, its output varies depending upon its application. The torque output of an automotive engine mainly depends on its stroke-to-bore ratio, compression ratio, combustion pressures & speed in rpm. Most ‘under-square’ engines which have higher stroke-length than its bore diameter, tend to develop the high amount of ‘low-end torque’. The amount of torque that an engine can exert depends upon the engine RPM.
Different engine designs/configurations develop different torque characteristics such as peak curve/flat curve. Most automotive engines produce useful torque output within a narrow band of the engine’s entire speed range. In petrol engines, it characteristically starts at around 1000-1200 rpm and reaching a peak in the range of 2,500–4,000 rpm. Whereas in a diesel engine, it starts at around 1500-1700 rpm and peaking at 2000-3000 rpm. Bugatti Veyron is one of the cars with the highest torque figures.
How to calculate engine torque:
If you know the Horse-Power of the engine, then you can use the following formula -
Torque= 5252 x HP/RPM
Why is engine torque important?
Torque and Horse-Power are the twin outputs of an engine. They are related and proportional to each other by speed. The 'torque-band' in an engine curve represents its pulling ability which determines a vehicle’s 'driveability' & 'acceleration'. Torque is most needed while moving a vehicle from the stand-still and/or climbing a slope. Similarly, heavier is the vehicle or a vehicle with full rated load requires a higher amount of torque to pull it and get it moving. In a conventional engine, the horsepower governs vehicle’s top speed (thru’ gear ratios) whereas torque controls its acceleration/pick-up. The rate of acceleration also depends on the vehicle’s weight and the ‘load’ carried by the vehicle.
Flat-Curve vs Peak-Curve engine torque:
Most petrol engines normally produce a considerably high amount of ‘low-end-torque’. However, usually, they exhibit ‘peak-curve’ torque in the shape of the ‘peak’ of a hill. In ‘peak-curve’ design, the torque peaks at the middle of the engine rpm range (around 2500-3000 rpm). After this, it starts to fade out rapidly while the horsepower still keeps rising. The HP reaches its maximum value later at a higher engine rpm and then fades out at the red-line.
Most modern diesel engines deliver a ‘flat-curve’ torque. In ‘flat-curve’ design, the engine produces maximum torque at a ‘lower-to-middle-end’ of the engine speed i.e. approx. 1500 rpm onward. Its value remains almost the same or ‘flat’ across most of the engine speed range (2500-4000 rpm). This helps in better acceleration and effects fewer gear shifts while driving.
What is Low-End Torque?
Often manufacturers use this term to describe an engine’s torque performance. ‘Low-End-Torque’ is the amount of torque that engine produces at the lower engine rpm band i.e. between 1000-2000 rpm. This rpm band is very crucial when moving a vehicle from stand-still or driving in slow-speed conditions such as in traffic. If the engine generates a greater amount of torque at the lower end of the rpm band, it implies that the engine has higher ‘low-end-torque’ or better pulling ability at slow speeds. It also means that the engine can move the vehicle quickly from stand-still, pull heavier loads or climb a slope relatively easily as the case may be without revving hard.
Engine Torque and Efficiency:
The engine torque reaches its peak value at a speed where it is most efficient. In other words, the engine efficiency is at the maximum at a speed where it produces its peak-torque. If you raise the engine above this speed, its torque starts to decrease because of the increased friction of the engine’s moving parts. So even if you rev the engine over & above the peak-torque speed, the torque doesn’t increase any further.
Engine torque is multiplied by gears. Lower the gear selected (i.e. 1st gear which has a high gear ratio), greater is the pulling ability of the engine. Therefore, the vehicle’s pulling ability is highest in the first gear. However, if you rev the engine further in 1st gear, it reaches its limit after some time; thereby prompting the driver to shift to the next gear. In contrast, if you change gears before the engine torque reaches its ‘peak’ value, the vehicle might lose its acceleration. This is because the wheels would not get enough force to rotate. Thus, compelling the driver to shift back to the previous/lower gear.
Engine Torque and Driving:
Best fuel efficiency can be obtained by changing the gears within the vehicle’s ‘Power-Band’ and changing gears as close to the peak torque value as possible. Also, to get better efficiency, select the correct gear/s corresponding to the vehicle speed/engine rpm as recommended by the vehicle manufacturer.
1. Highway Scenario:
Topmost available Gear (i.e. 5th or 6th or so on) + Lowest Engine Speed = Best Fuel Efficiency
2. When climbing a slope/gradient:
Low gear (i.e. 1st) + High engine speed = Least Fuel Efficiency but more pulling ability.
Once your vehicle goes past 60 Km/h such as on the highway, you do not need high engine rpm to keep it going. This means that while cruising on highways/motorways, use the topmost gear and keep the engine rpm to below 2500 to get maximum efficiency. Similarly, while climbing a slope you need to use the lower gear (i.e. 1st gear) and higher engine rpm to pull the vehicle (and load if any) against the force of gravity. However, it will affect fuel efficiency.
These values are mentioned in every Owner’s Manual. Having said this, always running the engine on ‘max-power/speed’ or revving the engine to the ‘Red Line’ zone is not necessary unless you are running a race as it will only result in burning of extra fuel.
Remember that such extra fuel either burned or saved will make a lot of difference at the end of the journey - be it short or long…!!!
Read more: What is Horse Power?