How is engine Torque measured

Torque is one of the most important and fundamental measurements used in automotive engineering. It represents the twisting force that an engine generates to rotate the vehicle’s wheels and perform work. Torque is crucial not only because it plays a key role in a vehicle’s acceleration but also in its ability to move heavy loads, climb hills, or maintain speed. However, understanding how torque is measured and the differences between various methods can be complex.

1. What is Torque?

Torque is a measure of rotational force and is typically measured in units such as foot-pounds (ft-lb) or Newton-meters (Nm). In the context of an internal combustion engine, torque is the force produced by the engine’s combustion process that causes the crankshaft to rotate. This rotating force is what moves a vehicle forward.

While horsepower measures the rate at which work is done, torque is more directly related to how much force the engine is producing at any given moment. Torque is what provides acceleration, towing capacity, and overall drivability. High torque at low RPM (revolutions per minute) means that an engine can generate substantial force at lower speeds, which is essential for activities like towing or off-road driving.

2. Methods of Measuring Engine Torque

There are several methods used to measure engine torque, each with its own advantages, limitations, and appropriate applications. These methods include direct measurements, indirect measurements, and theoretical calculations. Understanding how each method works is essential for interpreting torque data accurately.

2.1 Dynamometer (Dyno) Testing

One of the most accurate ways to measure engine torque is through the use of a dynamometer (dyno). A dyno measures the power and torque produced by an engine by applying a load to it and measuring the resulting force.

2.1.1 Engine Dynamometer (Engine Dyno)

An engine dynamometer measures torque directly at the engine’s output shaft, typically at the crankshaft or flywheel. The engine is removed from the vehicle and mounted on the dynamometer, where it is connected to a load cell or a braking system. The dynamometer applies a controlled load to the engine and measures how much torque the engine can produce to overcome this load.

There are two main types of engine dynamometers:

Water Brake Dyno: This type uses a water-based system to create resistance. Water is circulated through a turbine or rotor, and the force required to turn this rotor is measured. The torque produced by the engine is calculated based on the force required to rotate the turbine.
Inertial Dyno: This dyno uses a large rotating flywheel to create resistance. The engine accelerates the flywheel, and the amount of energy needed to increase the speed of the flywheel is used to calculate torque and power.

Pros of Engine Dyno Testing

Direct Measurement: Engine dynos measure the engine’s raw power and torque without the influence of drivetrain losses, making it the most accurate way to assess engine performance.
Controlled Conditions: Testing takes place in a controlled environment, meaning external factors like road resistance, temperature, and air density are minimized.
High-Precision: The precision of a dynamometer allows engineers to measure torque very accurately, even at different RPMs, which is useful for tuning and optimizing engine performance.

Cons of Engine Dyno Testing

Not Real-World Representation: Because the engine is removed from the vehicle, it doesn’t account for the effects of the drivetrain, tires, and road conditions, which can influence how torque is transmitted to the wheels.
Labor-Intensive: The process of removing and setting up the engine on a dynamometer is time-consuming and can be costly.
Does Not Reflect Full Vehicle Performance: It only measures the engine’s output and does not account for factors like transmission losses, which can affect torque at the wheels.

2.1.2 Chassis Dynamometer (Chassis Dyno)

The chassis dynamometer measures torque at the wheels, which is a more practical way of understanding how the engine’s torque is translated into usable power in real-world conditions. In this setup, the vehicle is driven onto rollers that simulate the road surface. The wheels are placed on the rollers, and as the vehicle accelerates, the rollers measure the torque transmitted through the drivetrain to the wheels.

Pros of Chassis Dyno Testing

Real-World Data: The chassis dyno measures the torque at the wheels, which provides a better understanding of how the vehicle performs on the road.
No Need for Engine Removal: The vehicle remains intact, so no time is wasted on removing and reinstalling the engine.
Considers Drivetrain Losses: Since the test accounts for the losses in the drivetrain, including the transmission, differential, and other components, it provides a more accurate representation of the vehicle’s overall performance.

Cons of Chassis Dyno Testing

Drivetrain Losses: Chassis dynos measure power and torque after the drivetrain has absorbed a portion of the engine’s output. This means that the torque measurement at the wheels will always be lower than the torque at the engine’s output shaft.
Influence of External Factors: The results can be affected by tire pressure, tire type, and even the road surface used in the test.
Less Control Over Variables: Unlike engine dynos, which test in a controlled environment, chassis dynos can introduce variations due to the complexity of the drivetrain and vehicle weight.

2.2 Strain Gauges for Torque Measurement

A strain gauge is a sensor that can be used to measure torque directly. It works by detecting the deformation (strain) of a rotating part, such as a crankshaft or drive shaft. As the engine applies torque to the shaft, it causes slight deformation, which can be measured using a strain gauge affixed to the part. The strain is proportional to the amount of torque being applied, allowing for a direct measurement.

2.2.1 How Strain Gauges Work

Strain gauges are typically installed on the crankshaft or other rotating components. When torque is applied, the shaft deforms slightly, causing the resistance in the strain gauge to change. This change in resistance is directly related to the amount of torque being applied. The sensor sends this data to a computer, which calculates the torque in real-time.

Pros of Strain Gauge Measurement

Direct Measurement: Strain gauges provide a direct measurement of the torque produced by a rotating component.
High Precision: When properly calibrated, strain gauges are very accurate and can measure small changes in torque.
In-Situ Measurement: Strain gauges can be used to measure torque while the engine is still installed in the vehicle, providing real-time data during operation.

Cons of Strain Gauge Measurement

Installation Complexity: Installing strain gauges requires precision and may involve modifications to the engine or drivetrain components.
Cost: The equipment and setup can be expensive, especially if the system needs to be calibrated and maintained regularly.
Sensitivity to Vibration: Strain gauges are sensitive to vibrations and other external factors, which could affect the accuracy of the readings if not properly managed.

2.3 Torque Calculations (Theoretical Measurements)

In some cases, especially when dyno testing or physical measurement tools are unavailable, torque can be estimated using theoretical calculations. These calculations rely on other data points, such as engine displacement, RPM, and the engine’s efficiency. For example, torque can be estimated based on the engine’s power output using the formula:

Torque=Horsepower×5252RPM\

{Torque} = {Horsepower} \5252\{RPM}Torque=RPMHorsepower×5252

Where:

Horsepower is the engine’s power output.
RPM is the engine speed.
5252 is a constant used to convert the horsepower and RPM into torque.

These calculations are commonly used to estimate the torque for engines that are still in development, or when physical measurement is not possible. However, these theoretical calculations may not account for factors such as heat loss, friction, and other real-world inefficiencies that can impact torque output.

Pros of Theoretical Torque Calculations

Quick Estimation: Calculations provide a fast way to estimate torque without requiring a dyno or other specialized tools.
No Need for Physical Measurements: This method doesn’t require removing the engine or using any physical sensors, making it ideal for early-stage engine development.

Cons of Theoretical Torque Calculations

Accuracy Issues: Theoretical calculations may not reflect real-world variables like friction, fuel quality, and mechanical losses.
Limited to Horsepower Data: These calculations rely on horsepower data, which itself is not always accurately measured in real-world conditions.

2.4 OBD-II Torque Data

Modern vehicles with an On-Board Diagnostics (OBD-II) system can provide real-time data about the engine’s torque via sensors integrated into the vehicle’s engine control unit (ECU). Many OBD-II tools and apps can read data from the ECU and provide estimates of engine torque based on a variety of parameters such as throttle position, RPM, and fuel efficiency.

While OBD-II torque readings are generally estimations based on sensor data, they can provide valuable insight into how the engine is performing under various conditions.

Pros of OBD-II Torque Measurements

Real-Time Data: Provides immediate feedback about the engine’s performance without needing to remove the vehicle or install additional sensors.
Cost-Effective: Many OBD-II scanners are relatively inexpensive and can be used with a wide variety of vehicles.

Cons of OBD-II Torque Measurements

Estimation-Based: OBD-II readings are based on estimates and sensor data, which may not be as accurate as direct torque measurements.
Dependent on ECU Calibration: The accuracy of OBD-II torque readings depends on the vehicle’s ECU calibration, which may not always be precise.

3. Conclusion

Measuring engine torque is critical for evaluating engine performance and understanding how an engine behaves under various conditions. Each method for measuring torque—whether through dynamometers, strain gauges, theoretical calculations, or OBD-II data—has its own strengths and weaknesses.

Dynamometers provide direct, real-world measurements of torque and power, but each type (engine vs. chassis) offers different insights.
Strain gauges offer highly accurate measurements in real time but require careful installation and maintenance.
Theoretical calculations provide estimations based on known data but can lack the precision of direct measurements.
OBD-II sensors provide quick, accessible data for everyday diagnostics, though these are based on estimated values.

Ultimately, the choice of measurement method depends on the application, budget, and desired accuracy. Whether tuning an engine for maximum performance, testing a new vehicle, or diagnosing an issue, understanding torque measurements is crucial for optimizing engine efficiency and performance.