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Post Info TOPIC: Engine Efficiency


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Engine Efficiency

Hello people,

The efficiency of an engine may be defined as the ratio of the "power" obtained, to the "power" supplied. In our gasoline-engines, there are many losses, so that in relation to the inherent "power"in the fuel ( stored liquid energy ), only about 15% appears as useful power to propel our vehicles...The rest is lost in the cooling-system, exhaust-system, engine friction & drive-line friction. To give some "general" but very-real numbers as to where most of our power is lost, see below...

* 35% is lost as rejected-heat-energy in our cooling systems

* 35% is lost as rejected-heat-energy in our exhaust system

* 10% is lost in drive-line friction

* 5%   is lost in engine-friction

* 15% is left to propel our vehicles

So looking at these numbers, it can be seen that there are some other "components" of our vehicles that we "can" improve-on in some way, to shift some percentage-points over into the "propel-our-vehicle" section !




"Only an alert & knowledgeable citizenry can compel the proper meshing of the huge industrial & military machinery of defense with our peaceful methods & goals, so that security & liberty may prosper together".    Dwight D.Eisenhower.


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Well gee... When I run alcohol I dont even need to put coolant in my engine if I'm only going 1/4 mile trips at a time.

Perhaps I should build an engine that has a magnetic field for a bearing and solenoids to open a close the valves?

By that time we will be ready for Mr. Fusion,

But no really, with all the modern advancements they are just now scratching the surface in terms of all the energy that is literally wasted. I guess thats what they call evolution.

For example in the lastet issue of Car and Driver the new Porsche 918 makes use of exhaust gas pressure by spinning both a turbo and a generator to recharge it's battery. Lot's of new ideas coming down the pipeline, but most of them too expensive to even consider at the moment.

-- Edited by SELLC on Thursday 4th of July 2013 07:10:21 AM


A middle-aged loner on a crusade to champion the cause of the innocent, the helpless, the powerless - in a world of criminals who operate above the law.




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LOL ! I just thought it interesting for people to know about how much wastage "has" to go on for our engines to run, & thought we could all throw some ideas around to see how these things might be improved. It seems that the more extra items that you can run from electricity is the way to go for now. ( eg engine-fans ).




"Only an alert & knowledgeable citizenry can compel the proper meshing of the huge industrial & military machinery of defense with our peaceful methods & goals, so that security & liberty may prosper together".    Dwight D.Eisenhower.


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Hello people,

It makes sense to me to put up in this next post a couple of important "understandings", so that we all know what Horse-power, Torque & a number of other "data - yardsticks" actually mean, where they come from, & how it relates to us & our vehicles. We have to start from the very basics, but hopefully you'll get a much clearer understanding of what Horsepower & Torque really relates to as measurements. So if your interested ( I hope it's not too boring ) keep reading.


When an object is moved from one position to another, work is said to be performed. Work is measured in units of foot-pounds, (ft.lbs). As an example, if a 3-pound weight is lifted 2-feet, the work performed would be 3 lbs x 2 ft = 6 ft. lbs. In other words, work equals the force in pounds required to move the object times the distance moved in feet. Work is performed when weights are lifted, springs compressed, shafts rotated.

The ability or capacity to do work is known as energy. A lump of coal or quart of gasoline has energy stored in it, which when released will perform work. A valve-spring can do the work of closing a valve when it is released after having been compressed.


Power is defined as the rate or speed at which work is performed. One horse-power is defined as the "amount" doing 33,000 ft.lbs. of work in one-minute. The unit of measurement was was originated by an engineer by the name of Watt, who found that a strong horse could hoist 366lbs of coal up a mine-shaft at the rate of one-foot-per-second. In one minute, the horse would have raised the 366 lbs. 60 feet. This would be the equivalent to raising 21,960 lbs., one foot in one minute. Arbitrarily, Mr. Watt raised this figure to 33,000 lbs. one-foot, in one-minute.

Expressed as a formula...

HP = per-minute / 33,000 = D x W / 33,000 x t

where D = the distance the weight is moved.
W = force in pounds required to move the weight through that distance.
t = time in minutes required to move the weight through the distance D.

For example....How many horse-power would be required to move a weight of 5,000 lbs., a distance of 60 ft., across a level floor in 3-minutes ?

HP = D W / 33,000 t = 60 x 5000 / 33,000 x 3 = 3.03 hp.


Torque is defined as turning or twisting effort. While torque is measured in pound-feet, it differs from "Work" or " Power" as torque does not necessarily produce motion. For example; if a 50 pound force was applied at the end of a 3-foot-lever, there would be 150 lbs. ft. of torque. Whether the lever moved or not would be beside the point.


Experiments have shown that dividing the force required to slide one object over another at a constant-speed, by the pressure holding them together is a "constant" which is known as the "coefficient of friction". The "coefficient of friction" is always the same for those materials & surfaces. For example, if a pull of 50 lbs is required to pull a weight of 120 lbs over a surface at a constant-speed, the coefficient of friction would be...

Cf = 50 / 120 = 0.416

Remember though that more force is required for initial movement than to keep the object moving. Sliding friction is therefore measured after motion has started.

A lubricant is a substance placed or injected between 2-surfaces to reduce friction. The thin-layer of lubricant, adhering to the 2-surfaces is then "sheared" by the movement. The friction within the lubricant being less than that between the 2-surfaces, means less force is required to produce movement.


No naturally-aspirated engine is 100% efficient. One of the factors affecting the efficiency of our gasoline-engines is the difficulty of getting a full-charge of combustible mixture into the cylinder, because of restrictions of the intake-manifold, atmospheric temperature, valve-timing & similar factors, a theoretically full-charge does not reach the cylinder. The ratio of the amount of charge actually taken in per cycle to a complete charge is known as the volumetric efficiency.

After a certain engine-speed is reached, the volumetric efficiency drops off rapidly. In general, maximum volumetric efficiency is reached at approximately the same point where maximum Torque is reached. As an example, say a 5.0 litre engine reached a maximum volumetric efficiency of 82% @ 3,000 rpm, but at 4,800 rpm, it may drop to only 65%.

As atmospheric-pressure drops with increase in altitude, volumetric efficiency will also decrease, as it is the difference in pressure, between the pressure outside the cylinder, & the pressure inside the cylinder, that determines the amount of mixture which will enter the cylinder.

In the next post, I'll try to further clear the fog with regards to all the various "HP's" that have been applied to our engines and their apparent out-put ratings, & what they really mean.




"Only an alert & knowledgeable citizenry can compel the proper meshing of the huge industrial & military machinery of defense with our peaceful methods & goals, so that security & liberty may prosper together".    Dwight D.Eisenhower.


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Hello people,

It looks like a few folks are taking the time to read through all of this stuff, so I thought I'd continue on with some more "fundamental" facts about our engines & measurements, & follow on with some even-more basics to help the "unsure people" get a better grasp of things. Once again, I hope it's not too boring, & at least provides the real-deal as to how things are analysed etc.


The diameter of an engine cylinder is referred to as the Bore, & the distance the piston moves from Bottom Dead Centre to Top Dead Centre is called the Stroke. Displacement of an engine is a measurement of its size & is equal to the number of cubic inches the piston displaces as it moves from BDC through to TDC. In other words, it is equal to the area of the piston times the stroke. In the case of multi-cylinder engines, it is also necessary to multiply by the number of cylinders.

Displacement = A x S x N...Where....

A = The area of the piston in square inches.
S = The length of stroke in inches.
N = The number of cylinders.

Let's assume that we have an 8-cylinder engine with a 4.0" bore, & a 3.0" stroke. The procedure is to first calculate the piston area...

The formula is TT x D / 4.

TT = Pye or 3.1416.
D = Piston diameter ( squared ).

So we have 3.1416 x (4.0 x 4.0) / 4 = 12.566 sq. in.

Then the displacement equals... 12.566 x 3.0 x 8 = 301.593 cubic inches... ( Alas our 302 cube V-8 !!! Or if you rather our 5.0 ltr V-8 ).


The Compression Ratio of an engine is the extent to which the combustible gasses are compressed within the cylinder. It's calculated by dividing the volume existing within the cylinder with the piston at BDC, with the volume of the cylinder with the piston at TDC. For example, if the volume with the piston at BDC is 45, & the volume with the piston at TDC is 5 ( within the combustion chamber ), then the compression ratio is...

45 / 5 = 9 to 1.

In other words, the gas is compressed to one-ninth its original value.

Up to a certain point, the more the fuel-charge is compressed, the more power will be produced. Experiments made by General Motors Engineers indicate that a 17 to 1 compression ratio is the peak efficiency for gasoline engines...However, with to-days fuel standards & component technologies, around the 12 to 1 ratio is considered to be the very "general" safe maximum, until further advancements are made, if that even eventuates.

In the real-world, the compression ratio can be increased by planing the material from the cylinder-head, installing thinner head-gaskets, increasing the stroke or by increasing the bore size. However, consideration must also be given to the possibility of valves hitting pistons. In case it's desired to increase the compression ratio of an engine, the following formula can be used...

B / C-1 = A where...

A = the volume of the combustion chamber.
B = the displacement of the cylinder.
C = the desired compression ratio.

For example, if the displacement is 36, & the desired compression ratio is 10 to 1, then...

36 / (10 - 1) = 4

In other words, for this particular engine, the combustion chamber would have to have a volume of 4 to obtain the desired 10 to 1 compression ratio. Don't forget that your piston-crown can also be considered part of the combustion chamber, & its shape / design could be the first point of consideration.


Brake Horsepower may be defined as the power that is available for propelling the vehicle. It is the power that remains after the effects of friction & the power that is required to drive the fan, water-pump, power-steering pump, oil-pump & alternator is subtracted from the power developed within the cylinder, ie., the Indicated Horsepower.

The term Brake-horse-power is derived from the equipment first used to determine the power developed by an engine, which is known as the Pony-Brake. This Pony-brake consisted of a large drum & a band-type-brake which operates on the outer surface of the drum. Attached to the brake is a lever, with its free-end bearing upon a weighted scale. The drum is connected directly to the engine crankshaft to be tested. As the drum is rotated, the brake is tightened, imposing a load on the engine, which in turn, causes the lever to be pressed against the scale.

When making a Pony-brake test, the throttle is first set to operate the engine at some specific speed. The brake is then tightened until the speed drops-off. The weight on the scale is then noted. This procedure is repeated, each time at a higher speed ( usually in increments of say 100 rpm ). The following formula is then used to calculate the brake-horse-power at speed.

BHP = 2 TT L R W / 33,000 = L R W / 5252 where...

L = length of lever-arm in feet.
R = engine speed in rpm.
W = load in pounds on scale.

The data obtained can then be plotted on your typical "x-y" scale graph, where "x" may be power, & "y" may be rpm. The same graph outlay is often used to plot Torque output etc.

Brake-horse-power can also be measured on ( you guessed it ) a Dynamometer. This equipment will usually consist of a resistance-creating-device, such as an electric generator / alternator, that is so arranged as to absorb & dissipate the power produced by the engine ( heat ). Suitable gauges are provided to indicate the amount of power absorbed.

In the testing labs, the engines can be directly connected to the shaft of the dynanometer. When used in "Service-stations", the dynamometer is provided with rollers that are then driven by the wheels of the vehicle. This drive-on type of dynamometer is often used extensively in diagnostic centres to provide factual & "near-real-world" information for the improvement of performance of the vehicle.


As previously described, torque is turning effort, & in the case of an automotive engine, the pressure on the piston provides torque. In designing an engine, engineers try to have the engine maintain as high a torque as possible through-out the speed-range of the engine. However, as engine-speed increases, there is less-time for the fuel mixture ( or just air in fuel injected vehicles ) to reach the cylinders due to "inertia" of the mixture, with resistance to its movement offered by the induction-system & valve-timing. As a result, Volumetric Efficiency is reduced & the torque is similarly reduced.


The rated horse-power of an engine is based on a formula developed in the "early-days" of the industry & is based on the assumption of a "brake-mean-effective-pressure" of 67.2 psi & a piston-speed of 1000 feet-per-minute. Today's engines operate at much-higher speeds & pressures & consequently the formula no-longer gives any indication of the power out-put of an engine. It is often incorrectly referred to as the SAE horse-power, but the correct name is "Rated" or "AMA" horse-power after an automotive association that is no longer in existence. However, the formula is still known to be used in different parts of the world for purposes of liscensing or registering vehicles. The formula is as follows...

Rated Horse-power = N B / 2.5, where...

N = the number of cylinders.
B = the is the diameter of the cylinder bore in inches - ( squared ).

So, consider an 8-cylinder engine with a 4.0" bore...

Rated Horse-power = 8 x ( 4.0 x 4.0 ) / 2.5 = 51.2 - This figure is also known as the "taxable" horse-power rating of an engine.

Oh well people, only a couple of more "HP's" to cover LOL, but I'm going to take a break & let you all stew on what's already posted. The remaining ones will hopefully complete the whole picture, & show how everything so-far posted is all inter-related.



-- Edited by Rastus on Monday 15th of July 2013 12:25:21 AM


"Only an alert & knowledgeable citizenry can compel the proper meshing of the huge industrial & military machinery of defense with our peaceful methods & goals, so that security & liberty may prosper together".    Dwight D.Eisenhower.


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Posts: 3845

Hello people,

Hear comes the last couple of installments about the facts & figures that are used to determine / rate the outputs that our engines produce. There will be other methods as-well that I'll say something about towards the end.


Another method of rating an engine is by the Indicated Horsepower. This is based on the actual power developed in an engine from an indicator diagram. As the indicated horsepower is the power produced within the engine, it includes the power required to overcome the friction within the engine. Subtracting the Friction Horsepower from the Indicated Horsepower also gives the Brake Horsepower.


The Indicator diagram is obtained by means of an oscilloscope or a special instrument ( planometer is used to provide a "draw-card-diagram" ) which makes an actual drawing of the events that are occurring in the cylinder. It records in diagram form, the pressures existing at each instant of a complete cycle of the engine from the time that the combustible mixture is first drawn into the cylinder until the end of the exhaust-stroke. The area of the diagram is then proportional to the power developed, ie, it is the Indicated Horsepower.

When calculating the Indicated Horsepower, it's first necessary to determine the Mean Effective Pressure which is the average pressure during the power-stroke, minus the average pressure during the other 3-strokes of the cycle. The Indicated Horsepower is then found / calculated using the following formula.

IHP = PLANK / 33,000 where...

P = Mean effective pressure in psi.
L = Stroke in inches.
A = Area of cylinder in
N = Number of power-strokes per minute. ( Think rpm / 2 for a 4-stroke engine ).
K = Number of cylinders.

NB. This method is often used on the larger Diesel Engines that have the fitting of "indicator-cocks" that allow you to "blow-through" the engine with the cocks open so as to provide a visual of a cylinders contents before start-up, as-well as for taking these diagrams. They're basically a valve that allows access to your engines cylinder.


Friction horsepower is the power required to overcome the friction within the engine. The friction results from the pressure of the piston & rings against the cylinder walls, friction of the crank-shaft & camshaft rotating in their bearings, & the friction of the other moving parts such as the oil-pump, fuel-pump, the engines valves, timing-gear & chains etc.

Friction horsepower increases with the speed of the engine & also with the Size of the engine.


So we can see that there are many different methods available of defining the output of our engines. There are some other methods, but I've tried to stick with the "industry-standards" that have known to be used or are still in use to this day. There's one other formula that I've found very accurate since it combines all the "elements" that are around us at the time of testing / proving, & it involves your whole car working in the "real-world" in "real-time". I've already posted it in the " What does a stock 560SEL do in the 1/4 mile ?" thread, & I'll post it again here. However, how do you folks think that your vehicles should be measured ? eg.

* By how fast your car actually goes ? eg top-speed.
* By how long it takes to get to top speed ?
* By what time it can produce in a 1/4 mile race ?
* By how steep a gradient it can accelerate up in top-gear ?
* By how long it can handle high-speeds before things go pear-shaped ?
* By how well it handles & accelerates around a race-track ? etc etc etc

As you can see, there's a lot of different ways to look at the out-right performance of our cars, & some people will prefer one method of performance to the other, for their own valid reasons, & really, there is no definitive way to define "performance" except in terms of how many boxes it ticks in the real-world that suits yourself & your own needs. But....I would say that we all like to get thrown-back in the seat when WOT is applied, & one way of determining reasonably accurately how well your car does this, is by using the general language that everyone around the world understands, & that's how much Horsepower it makes !

This next & final formula will place everyone on the same playing field, as everyone is exposed to the same environment, & ultimately determines the outright performance of your engine & drive-line within the whole package of your car in the real-world, & not by what the "lab-tests" necessarily point to, though in the lab, the results are accurate & true. This formula is in metric for greater accuracy, & is easily calculated from Kilo-Watts to Horsepower by multiplying the final result by 1.34.

Power = Mass x Gravity x Friction-force x Distance / Time.

Power = Kilo-Watts = Kw
Mass = Kilo-grams = Kg = the weight of your car with yourself & fuel included.
Gravity = Newtons = N =9.81
Friction Force = Ff = 0.4 = coefficient & has no units as calculated earlier in the posts.
Distance = 1/4 mile = 400 meters = 0.4 Kilometers = Km
Time = Seconds = your result.

As an example, let's use the 560 SEL again as the figures spring to mind....

1800 x 9.81 x 0.4 x 0.4 / 15.1 = 187.104 Kw

Multiplied by 1.34 to get HP = 187.104 x 1.34 = 250.7 HP.



-- Edited by Rastus on Monday 15th of July 2013 10:31:35 PM


"Only an alert & knowledgeable citizenry can compel the proper meshing of the huge industrial & military machinery of defense with our peaceful methods & goals, so that security & liberty may prosper together".    Dwight D.Eisenhower.

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