How to Measure the Real Power of a Combustion Engine
Knowing the real horsepower of a car goes beyond the figure published by the manufacturer. This value, expressed in HP or kW, is obtained under ideal laboratory conditions, without considering all the components that later consume energy. 🏎️
The Fundamental Power Equation
In a thermal engine, power defines how much work it can generate in a given time. To obtain it, you need to know the engine torque and the revolutions per minute. The relationship is expressed with this formula: Power (HP) = (Torque (N·m) x RPM) / 7021.36. An engine can be powerful by generating high torque at low RPM or by reaching very high engine speeds. The power curve illustrates this behavior across the entire useful range.
Key Factors in the Calculation:- Engine Torque: It is the torsional force exerted by the crankshaft. It is measured in Newton-meters (N·m).
- Revolutions (RPM): The speed at which the engine rotates. Maximum power is usually reached at a specific engine speed.
- Delivery Curve: An engine with good torque at low RPM responds better in daily driving than one that only offers high power.
The declared power is usually measured at the crankshaft, with the bare engine and without accessories like the alternator or water pump.
Brake Power versus Wheel Power
This is where the big difference lies. Brake power is what the manufacturer measures directly at the crankshaft. Wheel power is what the wheels actually transmit to move the vehicle. Between both points, there is an inevitable loss.
Sources of Power Losses:- Transmission: The gearbox, driveshafts, and differential generate friction.
- Mechanical Resistance: Each gear and bearing absorbs part of the energy.
- Accessories: Elements like the power steering pump or air conditioning compressor consume engine force.
The Real Impact on the Road
These losses can subtract between a 15% and 25% of the original power. Therefore, a car advertised with 150 HP might be delivering only about 120 useful HP on the asphalt. When talking about performance, remember that not all horses pull the cart; some are lost along the way. 🔧