This is the second article in a 3-part series. Part 2 covers how engine efficiency is measured and classified, including the difference between thermal and mechanical efficiency, the various definitions of horsepower, and the standard testing methods used to rate engine output. Knowing where the energy from fuel actually goes helps explain why even a strong-running engine never delivers all its potential power to the wheels.
Thermal and Mechanical Energy Conversion
Internal combustion engines operate on the principle of converting chemical energy in fuel into heat energy, and subsequently into mechanical work. Efficiency metrics define the percentage of energy retained versus the percentage lost to parasitic heat transfer and friction.
Thermal Efficiency
This measures how effectively the engine converts the fuel’s heat energy (measured in Btus) into useful work. Average engine thermal efficiency is approximately 25 percent. The energy loss breakdown consists of 35 percent lost to the exhaust system, 34 percent lost to cooling and lubrication, and 6 percent lost to radiation and incomplete combustion. Brake Thermal Efficiency equals Brake Horsepower (bhp) times 33,000 divided by the weight of fuel burned per minute times the fuel heat value times 778.
Mechanical Efficiency
This is the ratio between the power developed inside the cylinders and the actual power delivered to the crankshaft. Standard tolerance is generally around 90 percent. Mechanical efficiency decreases as engine speed (RPM) increases due to increased internal friction. Mechanical Efficiency equals Brake Horsepower (bhp) divided by Indicated Horsepower (ihp).
Horsepower Definitions
Potential Horsepower is the total theoretical energy available in the fuel if 100 percent of heat were converted to work. Indicated Horsepower (ihp) is the measure of power developed within the cylinders. It is calculated by measuring the mean effective pressure (mep) using indicator graphs throughout the intake, compression, power, and exhaust strokes. Brake Horsepower (bhp) is the actual usable power delivered at the engine crankshaft after accounting for internal friction.
Component Relationships
An increase in average cylinder pressure directly correlates to an increase in Indicated Horsepower. As mechanical friction increases due to wear or high-speed operation, the delta between ihp and bhp widens, lowering mechanical efficiency. The Btu rating per pound of fuel directly dictates the upper limit of Potential Horsepower.
Work and Power Metrics
Energy developed within an engine is measured by the pressure exerted on the piston, while usable energy is measured by the torque delivered to the output shaft. The fundamental why of these measurements is to quantify internal pumping losses (friction and heat) versus external work capacity.
Indicated Horsepower (ihp)
The PLANK formula determines the theoretical power produced inside the combustion chamber by calculating the Mean Effective Pressure (mep) against the physical dimensions of the engine. ihp equals P times L times A times N times K divided by 33,000. In this formula, P is mep in lb per sq in, L is length of stroke in feet, A is cylinder area in sq in, N is power strokes per minute (RPM divided by 2 for a 4-cycle engine), and K is the number of cylinders.
Brake Horsepower (bhp)
Brake horsepower is determined by measuring the actual torque produced at the crankshaft or flywheel using a resistance device. The Prony Brake Formula states that bhp equals 2 times pi times R times L times W divided by 33,000, or bhp equals R times L times W divided by 5,252. In these formulas, R is engine RPM, L is length from the center of the drive shaft to the scale pressure point, and W is weight registered on the scale.
Measurement Methodologies and Component Relationships
A Prony Brake uses mechanical friction. A clamping device applies friction to a spinning flywheel. The force required to hold the flywheel at a specific RPM is weighed to calculate torque. An Engine Dynamometer uses electrical or fluid load. The engine drives a generator or dynamo. By varying the electrical load, engineers measure torque and bhp across different altitudes and atmospheric pressures. A Chassis Dynamometer uses roller loading. It measures bhp at the drive wheels and accounts for driveline losses including transmission, differential, and tires that are not reflected in engine-only testing.
System Logic and Constraints
Brake Horsepower is not a constant value; it fluctuates with engine speed. It is used to establish the engine’s performance envelope under Wide Open Throttle (WOT) conditions. Engine dynamometer testing often occurs in altitude simulation chambers. Changes in air density (pressure) directly affect the oxygen available for combustion, thereby impacting ihp and bhp. In both Prony and Dynamo testing, the load must be adjusted until a specific RPM is reached with the throttle at WOT to find the peak power points on the performance curve.
Force vs. Work
Engine Torque is the measure of twisting force or turning effort applied to the crankshaft. While torque measures the capacity to do work, Horsepower measures the rate at which that work is performed. Torque (twisting force) is calculated as Force times Distance. In U.S. Customary units, this is expressed in pound-feet (lb-ft). One pound of force applied to a one-foot lever equals one lb-ft of torque. Frictional Horsepower (fhp) represents the power lost to internal engine friction. It is the delta between the power generated in the cylinders and the power delivered to the crankshaft.
SAE Horsepower Rating Standards
The Society of Automotive Engineers (SAE) utilizes specific testing protocols to standardize output data across different atmospheric conditions and engine configurations. Gross Horsepower involves a bare engine that includes only essential components for operation such as the oil and fuel pump. This measures maximum theoretical output of the engine block design. Net Horsepower involves a full accessory load that includes the air cleaner, alternator, water pump, and full exhaust. This represents the actual power available to the consumer in a production vehicle. Standard Test Conditions (SAE) include a barometric pressure of 29.38 inches Hg (99.01 kPa), an ambient temperature of 85 degrees F (29.46 degrees C), and a humidity of 0.38 inches Hg water vapor pressure.
Component Relationships and Performance Variables
Frictional horsepower (fhp) increases as engine speed (RPM) increases. Higher surface speeds between moving parts such as pistons and bearings generate more resistance. A chassis dynamometer measures power at the drive wheels, accounting for additional losses through the transmission, differential, and tire rolling resistance.
Frictional Horsepower Calculation
Frictional horsepower (fhp) equals ihp minus bhp. In this formula, ihp is Indicated Horsepower (internal potential) and bhp is Brake Horsepower (delivered output).
Torque Calculation (Leverage)
Torque equals Force times Distance. In metric units, this is expressed in Newton-meters (N-m). In U.S. Customary units, this is expressed in pound-feet (lb-ft).
Engineering Constraints
Maximum torque point occurs when the engine achieves maximum air-fuel charge density. Increasing engine speed beyond this point results in diminished torque due to breathing restrictions, even if total horsepower continues to rise temporarily. Accessory load reduces net horsepower, which is significantly lower than gross horsepower because it accounts for the mechanical drag of engine-driven accessories such as alternators, emission controls, and cooling fans.
Once you understand how power is measured and where energy losses occur, you are ready to look at how the physical dimensions of the engine affect its performance. The 3-part series continues with Part 3. Proceed to Part 3.
Local Shop Note:
This is similar to something a technician on Genesee St in Chittenango, N.Y. told me about — a repair where the symptoms pointed one way, but the real cause was somewhere else. He was at an ATTS seminar, and he was telling me about a sedan that came in with a complaint that it had no power on hills and fuel economy had dropped significantly. The customer had already replaced the fuel filter, spark plugs, and oxygen sensors. Still had no power.
He checked fuel pressure — good. Checked compression — good. Checked ignition — good. Then he put the vehicle on a chassis dynamometer to measure brake horsepower at the wheels. The power curve was flat and low — the engine was making power, but it wasn’t getting to the wheels. He checked for dragging brakes — they were fine. Then he checked the transmission fluid — it was dark and smelled burnt. The torque converter lockup clutch was slipping, robbing power and generating heat. The engine was fine — the transmission was absorbing the power.
He rebuilt the transmission and replaced the torque converter, and the power came back with fuel economy restored.
After seeing enough repairs like that, you start to realize brake horsepower is what actually moves the vehicle. The engine might be making good indicated horsepower, but if the transmission is slipping or the driveline has high friction, that power never reaches the ground. Always verify the whole system when you’ve got a power complaint — sometimes the engine is doing its job, and the driveline is the weak link.