Engine Power Concepts

Home , Engine displacement, Mean effective pressure, Revolutions per minute

Stroke

C/R Stroke

PRODUCT INFORMATION

PRODUCT INFORMATION – ENGINE POWER CONCEPTS

CONTENTS

Introduction ...... 4

Engine Sizing ...... 5

Engine Power ...... 6-7

Brake Mean Effective Pressure ...... 8-9

Power Curves ...... 10-13

Summary ...... 14

3 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

ENGINE POWER CONCEPTS

Selling engines requires knowledge of their they refer to processes or relationships which capabilities and components. Potential are actually easy to understand — most relate to customers will have questions regarding size or power. These general terms apply to all applications and engine design features. While piston engines — Cat engines as well as the some of the terms used to describe various engine in your car. engine parts or functions can sound complex,

4 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

ENGINE SIZING

Bore refers to the inside diameter of the Displacement, or swept volume, per cylinder is cylinders in an engine. The piston is slightly the volume of air a piston displaces as it moves smaller than the bore measurement because it through one stroke. These terms are used slides in the cylinder. interchangeably. Both mean bore area times stroke. Stroke is the distance the piston travels in the cylinder. The length of the stroke is determined Bore Area = (3.14 x bore squared)/4 by the crankshaft radius also known as crank Displacement per Cylinder = Bore Area x Stroke throw (the distance from the centerline of main Engine Displacement = Displacement per bearing journal to centerline of connecting rod Cylinder x No. of Cylinders bearing journal). This movement is controlled by the shape of the crankshaft. displacement or Stroke swept volume

C/R Stroke C/R If the bore diameter and stroke are in inches, the displacement will be in cubic inches. If the bore diameter and stroke are in centimeters, The connecting rod connects the crankshaft to displacement is in cubic centimeters. 100 cubic the piston. As the crankshaft rotates through centimeters is one liter. 180 degrees, the connecting rod and the piston Compression Ratio is the ratio of volume in the move from the extreme bottom position (BC) to cylinder with the piston all the way down vs. the extreme top position (TC). The stroke then is all the way up. If the minimum volume in the two times the crankshaft crank radius (C/R). The cylinder with the piston at TC is one cubic inch crank radius is also the lever arm on which the and the maximum volume with the piston at BC force from the piston acts to produce torque. is 10 cubic inches, the compression ratio is 10:1. • Automotive gasoline engines have compression ratios between 7:1 and 12:1. • Diesel engines have compression ratios between 13:1 and 24:1. Generally, larger diesel engines have the lower compression ratios.

5 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

ENGINE POWER

Engine speed (the number of revolutions made by the crankshaft in one minute) is measured in rpm (revolutions per minute).

rpm Torque Lever arm

An engine producing 1000 lb-ft torque at 2000 Torque is the twist on a shaft resulting from a rpm, through transmission gearing can produce force applied perpendicular at a lever arm. Its 2000 lb-ft torque at 1000 rpm assuming no units are force (pounds) times distance from the efficiency losses through the transmission, for center of the rotating shaft (feet). Thus, 100 example. An increase in torque is achieved at the pounds applied at a lever arm of 2 feet results in expense of speed. The power in both cases is the 200 lb-ft torque. Equal torques can be produced same. To increase engine power we strive to by a large force applied at a short lever arm or increase torque (lb-ft) or speed (rpm) or both. a small force applied at a long lever arm. The torque from one-pound force applied at a 10-foot lever is the same as from a 10-pound 10 lbs force applied at a one-foot lever, etc. 1 lb

2 ft 1 ft

ue rq 10 ft o 4 lb-ft T 2 lbs Torque = 10 lb-ft

In an engine, pressure is applied to the top of the piston from expansion of an ignited air and fuel mixture. This pressure results in a force from the piston applied at the crank radius through the connecting rod. The resulting torque causes the crankshaft to rotate. By definition, work is force applied for a distance, or in the case of a rotational situation, work is torque applied through an angle. Power is work performed per unit of time.

6 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

ENGINE POWER

The most common unit of engine power in the Variables influencing power rating are: U.S. is horsepower (hp). Originally, this unit • Temperature of the air was derived by what an average horse could do. Rigged up with a pulley system, an average • Temperature of the fuel horse could lift 33,000 pounds one foot off the • Barometric pressure ground in one minute. • Humidity • Heat content of the fuel

The total horsepower actually developed on the pistons is called indicated horsepower. It is greater than the power measured at the engine flywheel by the horsepower required to Because power takes into account engine torque overcome frictional losses in the bearings, piston output as well as engine speed, it is a convenient rings, etc. as well as operating satellite systems unit used to compare engine size. such as fuel, oil, and water pumps. The Though horsepower is an accepted unit to rate difference between indicated horsepower and engines, each application must be considered flywheel horsepower is called friction individually. The engine ratings can be: horsepower. • Power that can be produced continuously The friction horsepower of an engine can be determined in the laboratory by motoring the • Power that can be produced for a given engine with an electric motor. In this test the time period, (generally one hour) followed engine’s fuel rack is at shut-off. The electric by an equal time period at a lower rating power required to motor the engine at any • Power that the engine can deliver for very given speed is the engine friction horsepower short times, such as five minutes at that speed.

7 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

BRAKE MEAN EFFECTIVE PRESSURE

Indicated horsepower less frictional Another way of viewing BMEP is that it horsepower equals brake horsepower. measures how effectively an engine uses its BMEP is a value referring to the constant piston displacement to produce torque. pressure which would have to exist in a cylinder • The higher the BMEP, the greater the during its power stroke to produce the same torque per unit of displacement. horsepower at the flywheel, as actually exists. • BMEP can only be compared between 4-cycle engine to 4-cycle engine and 2-cycle engine to 2-cycle engine. • Over the years, BMEP has become known as a measure of engine life, however, it is NOT.

+ • BMEP gives a fair indication of mechanical pressure stresses within the engine, but in no way is o indicative of thermal loads. compression power exhaust intake –

Pressure within the cylinder varies considerably. A rough indication of that pressure is shown above. You see that the pressure acting on the Example: One engine operating at the same piston varies considerably during the power speed (1800 rpm), but with varying turbocharger stroke. The mean or average pressure which boost and amount of aftercooling. would produce the same brake horsepower is the BMEP. Example Engine (1800 rpm)

BMEP

+ pressure o

compression power exhaust intake –

Column 1 — shows a naturally aspirated engine producing 100 hp at a BMEP of 84 Column 2 — light turbocharging greatly increases the air inlet temperature, but raises horsepower to 134 and BMEP to 119; fuel consumption has decreased 6% to .402 lbs./bhp hr., but internal pressures have increased 44%;

8 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

BRAKE MEAN EFFECTIVE PRESSURE because air inlet temperature has increased 171°, Conclusion — As BMEP is increased, fuel thermal loading has increased 19%. consumption falls consistently. Mechanical loadings due to cylinder pressures increase, Column 3 — an engine with the same degree of while thermal loadings rise slightly, then start to turbocharging as in column 2, but with moderate decrease. This demonstrates that BMEP, with aftercooling; air inlet temperature decreased to little or no direct correlation to either 200° F, although horsepower and BMEP both mechanical or thermal stresses, it is not an increased; fuel consumption again decreased; indication of engine life. because turbocharging is also the same as in column 2, maximum cycle pressure is also the • Properly designed high BMEP engines may same, while cooler inlet air lowers thermal have even better life expectancy than a loading. naturally aspirated engine. Column 4 — the same degree of aftercooling as • A high BMEP engine will have better in column 3, but with a light turbocharging bhp-hr production (i.e. total amount of boost; again bhp and BMEP increased, while work performed) than its low BMEP lowering fuel consumption; the higher boost counterpart. pressure brings considerably higher maximum cycle pressures; but thermal load remains almost A modern, naturally-aspirated, heavy-duty diesel unchanged from column 3. will live 10,000 hours between overhauls. For example, a moderately blown version of the Column 5 — a very high degree of aftercooling same engine will produce 35 percent more on the same amount of turbocharging boost as power for 8500 hours before overhaul, the blown column 4; Horsepower now stands at 214 — engine, at higher BMEP, has produced nearly 15 114% more than the naturally aspirated engine; percent more bhp hours than the naturally- BMEP is nearly 100 psi higher than that of the aspirated engine, using only about 10 percent naturally aspirated engine; fuel consumption is more fuel to do it (less, per bhp-hr.). more than 9% lower; maximum cycle pressure has increased, but less than horsepower has; Another way to look at it is that this moderately thermal loading is only 13% greater than that of blown engine would require only 7400 hours to the naturally aspirated engine, and is lower than produce the same 1,000,000 hp-hrs and would that of the lightly turbocharged, not aftercooled burn less fuel to do it. So, if an engine was engine in column 2. designed for that degree of turbocharging, it may actually outlive a naturally aspirated engine.

9 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

POWER CURVES

Engine torque can be measured using a If fuel consumption is measured for each dynamometer, which is a device allowing an loading, we can also produce a curve for this operator to vary engine load. With the engine data. Given in terms of the quantity of fuel running at full load speed, torque is measured burned to produce one brake horsepower for one and plotted. The load is then increased slightly, hour, this data is called Brake Specific Fuel and the torque is measured, along with the drop Consumption (BSFC), as shown below. in speed; and that point is plotted. A further load increase then produces further engine speed reduction and torque increase, and another point to graph. When enough points have been plotted, Torque we can connect them producing a lug torque curve as shown below.

hp 0.5 0.4 BSFC

Torque 0.3 rpm

The fuel consumed may be measured by weight (pounds in the English system; grams in the metric system) or by volume (gallons or liters).

rpm

Because horsepower is a straight mathematical derivation of the two quantities shown on the graph (rpm and torque), we can calculate a horsepower for each point on the torque curve, and arrive at a corresponding lug horsepower curve.

Torque

rpm

10 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

POWER CURVES

BMEP is also sometimes shown on such graphs before a load is applied. If the applied load is by calculating BMEP for each point on the equal to or less than that which the engine can torque curve and plotting the resulting data. carry at that throttle setting, the governor opens the rack enough to allow the engine to produce the required power, and engine rpm remains steady.

Torque BMEP

Lug Torque hp 0.5 Torque 0.4 BSFC 0.3 rpm

Lo-Idle Full Load For each engine setting, besides a lug torque curve, there is also a starting torque or rpm acceleration curve. When a load is applied to an engine which is operating considerably below If, however, a load greater than full load is full load rpm, and the engine must then applied, the engine will no longer be able to accelerate carrying that load, a curve similar to maintain steady speed at that governor setting, that shown below would be produced. This type and will begin to slow down, or lug. of loading is common in applications such as road vehicles.

Torque

Torque Torque Rise

Acceleration Torque rpm

Lo-Idle Full Load Because volumetric efficiencies are somewhat rpm better at the slower speeds and frictional losses are smaller, a greater torque can be produced at A completely different torque curve is produced lower speeds, so the increased load can be by an engine operating at or near full load rpm carried.

11 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

POWER CURVES

A certain amount of torque, or rise, is normal under lug conditions, but it can be substantially increased by modifying the fuel and air systems.

With modern, medium-speed, turbocharged, Lug Torque aftercooled diesels, this potential torque increase Torque is approximately 20 to 50 percent.

Lo-Idle Full Load Modified Curve rpm Torque This type of loading, with the engine running with a fixed throttle setting at or near full load, Normal Curve is common in earthmoving equipment, such as crawler tractors, track-type loaders, and many shovel applications. It is also the normal loading rpm on generator sets. By superimposing the acceleration and lug curves, we can see the two different basic torque If the torque rise is steep enough, the engine curves common to all engines. Actually, an may develop more power at the lower speed, infinite number of possible curves exist, because torque increases faster than speed depending on the engine speed at the start of decreases. loading, and the throttle opening.

Torque

Lug Torque

hp Torque

Acceleration Torque rpm

Lo-Idle Full Load The engine can support some overload, although rpm at a reduced speed. Should the applied load be greater than that shown at peak torque, the engine will rapidly slow down further, produce less and less torque, and stall.

12 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

POWER CURVES

Another curve you will often see is called a Pressure Time (PT) curve. The vertical axis represents pressure; the horizontal axis Injection represents time. Time in this case is measured in degrees of Pressure engine crankshaft rotation rather than in seconds or minutes. When the crankshaft has made one Ignition Delay full revolution, it has traveled 360 degrees. When the piston is at its lowest point in the cylinder, it is at Bottom Center (BC). As it starts BC TC BC upward on the compression stroke, pressure begins to rise, until it reaches a maximum when Time the piston is at Top Center (TC). Should that 2. When the fuel does start to burn, heat is cylinder fail to fire, pressure would drop off as generated, rapidly increasing the pressure shown, and would again be zero at BC. of the fuel-air mixture. 3. The peak pressure comes some few degrees after TC. Although some of the fuel is still burning at this point, the piston is moving down so rapidly that volume increases faster than the pressure can now increase, and Pressure pressure starts to fall off. The volume between the two curves is the net work produced by combustion. A long-time engineering goal is altering the shape of that curve so that the volume under it is maximized

BC TC BC for a given cylinder pressure. Time

If the cylinder does fire: 1. At some point shortly before TC (usually 10-20 degrees of crankshaft rotation), fuel injection begins. (This fuel is cold, and has Pressure not had time to mix with air, so it does not immediately start to burn.) This period is called ignition delay.

BC TC BC Time

13 PRODUCT INFORMATION – ENGINE POWER CONCEPTS

ENGINE POWER CONCEPTS

Understanding the basic concepts of how engines produce power is vital to successfully selling, buying, operating, or maintaining an engine for any application. The more knowledge you have, the more you contribute to ensuring an engine’s superior performance and reliability. Most of the terms associated with engine power are easy to understand. Some of the basic terms related to engine sizing Terms associated with engine power include: include: Horsepower (hp) — a measurement of engine Bore — the diameter of each cylinder in an power engine Torque — the twisting force engines produce Stroke — the distance a piston travels up and Brake Mean Effective Pressure (BMEP) — down within a cylinder the pressure in a cylinder required to produce Displacement — the volume of air which the the same horsepower at the flywheel as actually piston displaces as it moves one stroke exists; is NOT indicative of engine life as many believe Compression ratio — the relationship between the minimum and maximum volumes between Lug — a slowing of an engine occurring when the piston crown and the cylinder head its load is greater than it can support at a (i.e. volume at BC divided by volume at TC) particular governor setting Pressure-time curve — a visual representation of the pressure within the combustion chamber during an engine’s cycle