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Calculating Engine Power Outputs

12/03/2009 7:35 PM

Hi guys

Ive been reading a few posts on here and thought i would sign up and ask a few questions.

I am designing an engine, I would like to find a way of calculating the power output of the engine. I have drawin it all up in CAD (solidworks) so i know all the required dimensions. I know how much force in pounds is pushing down on the pistons. does anyone know how I could calculate the power out put of the engine?

Gareth

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#1

Re: Just an idea

12/03/2009 9:05 PM

For horse power the formula is (torque X RPM)/ 5252

If you can come up with the torque and the degrees of the rotation it applied for from each cylinder it can be roughly calculated for theoretical power.

However without knowing every detail and working parameter its almost impossible to say with any accuracy what it may actually do in real running conditions.

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#2
In reply to #1

Re: Just an idea

12/04/2009 8:11 AM

Thanks for the reply

How do i calculate the torque of the engine?

Is there a PDF anywhere that could explain this to me?

Gareth

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#3

Re: Just an idea

12/04/2009 9:35 AM

Have you never had a physics class?

pressure. area, length of a lever. That sort of basic common knowledge stuff?

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#4
In reply to #3

Re: Just an idea

12/04/2009 2:19 PM

yes I have

I am also working on a degree in physics (only started 2 months ago).

I Dont want you to do the work for me, I want help understanding how i go about calculating the power output of the engine.

Gareth

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#22
In reply to #3

Re: Just an idea

02/02/2010 3:22 AM

A truly useful answer.

Is anyone lacking what you consider "basic common knowledge" precluded for asking questions on this site?

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#5

Re: Calculating Engine Power Outputs

12/05/2009 12:54 AM

The problem is this; it is extremely difficult to determine the horsepower output of a engine design without actually building and testing the engine.

There are so many variables involved that this would require some serious computer processing time to get even an approximation.

It's certainly not impossible, but consider that major race engine builders (Formula 1 for example) employ supercomputers to perform these calculations. Even then, they don't always get it right.

I would suggest getting several race engine design books. There are some simple empirical equations which could generate a rough estimate, but a truly accurate answer will be difficult.

There are hundreds of variables which would impact horsepower output. It's FAR more than just the force pushing down on the pistons.

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#6

Re: Calculating Engine Power Outputs

12/05/2009 9:39 AM

Just because the answer to a question is complicated, is no reason to avoid it. The answer to your question is complicated and I suggest a reference book on the subject written by Charles Fayette Taylor by the name of "The internal Combustion Engine in Theory and Practice". It is a two volume set and has been used as a text at many universities. You will probably want to first skim it to determine the scope of engine variables such as fuels, volumeteric efficiency, two stroke v/s four stroke, heat loss, thermodynamics of actual working fluids, etc. You might become so fascinated with predicting engine performance that you make a career out of it, or you might give up and use a dynamometer. Either way, you will get a good understanding of the subject.

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#7

Re: Calculating Engine Power Outputs

12/05/2009 11:25 AM

Gareth: I have noticed that you have posted a variety of questions on various topics. All of them are characterized by 1) lack of information [a common fault with many posters to this forum]; 2) an obvious lack of basic understanding; and 3) an even more obvious lack of research. I don't say this to discourage you but to increase the value of the exchange to you and to the readers who are kind enough to attempt to answer your questions. The current question is typical. You do not tell us what kind of engine you are designing. Admittedly it does not matter because the scope of the question is so broad, but it leads to type of responses you have received. After prodding you admit that you are a student. Nothing wrong with that and as others have noted, it would be helpful if you say so up front and the purpose of the query. On the other hand, the specific question "how to calculate torque" can be answered by any 8th grade physics student. So I wonder what is going on here. If you are really serious I suggest that you take advice already given and start reading some of the basic texts cited. I would not start with racing engines however but fundamentals. I would also take a look at the SAE library. If you can prove that you are an enrolled student at a recognized university you can get free or discounted access to the library. regards

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#8

Re: Calculating Engine Power Outputs

12/05/2009 4:42 PM

I am curious, how you know how many pounds will be pushing on the pistons? At what RPM is "That" pressure being developed at? What type of valve layout are you using? Just wondering, thanks!

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#9

Re: Calculating Engine Power Outputs

12/05/2009 7:05 PM

Hi Gareth,

I'd suggest a copy of the Bosch Automotive Handbook as well as a text on automotive engine design.

Assuming your engine has a crankshaft, and is similar to existing ICE's, then you first may be able to estimate BMEP (brake mean effective pressure -- check wikipedia on this). This is the imaginary pressure that, if applied for the entire power stroke would produce a torque. It varies with engine specific output (so a low performance engine -- like a commercial or lawn mower engine has low BMEP, and a racing motorcycle engine has high BMEP). Then look in the Bosch book or elsewhere to go from BMEP to torque. Given torque and rpm, HP comes with a quick calculation (wikipedia horsepower).

If you cannot estimate BMEP, then you need to calculate the pressure available at intervals of the crank rotation. The heated mixture will cool from expansion, reducing its pressure, and will also expand (further reducing pressure) from the mechanics involved. You will find that where the leverage is best, the pressure is not the best -- a sad fact. Coming up with actual pressures at each increment is no small task -- and even getting remotely close requires a good understanding of thermodynamics and physics.

The flame front travel and heat generation are a dynamic processes in which the rate of reaction changes with pressure, and pressure changes with heat (the desired output of the process) and because the engine is rotating, there are the mechanical effects on pressure, and thus reaction speed, etc.

A solid background in calculus would be helpful, although it seems that it would be possible to use Excel (etc.) to look at discrete points and make summations of these discrete points. Challenging.

Looking at cylinder pressure charts of existing engines would give you an idea of what to expect -- again a textbook would be helpful here. I'd suggest John Heywood's book on fundamentals.

If you are certain you really know the force on the piston at each point in the power stroke, then it's just a matter of leverages and trig to find the integration of the forces over the power stroke. Make some subtractions for friction, for energy used in compression, etc. and you will come up with an estimate of power produced.

But certainly, if this is something not too far from an existing engine, then guesstimating a BMEP and working from there is the way to go. Or... depending upon your personality, just build the thing and test it.

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#10

Re: Calculating Engine Power Outputs

12/06/2009 12:48 AM

Gareth-

atsysusa (#7) makes some good points. Most of us don't mind answering questions that are fairly specific and focused, but- when you ask questions that SHOULD be solved by a quiick Google exploration or opening a basic handbook or text it detracts from the value of the forum.

Related to you question-

The BASIC (raw) horsepower formula is force (pressure per unit squared times area) acting over a distance (stroke in feet times number of pistons times RPM (divided by 2 if the engine is 4-stroke)) over time (one minute if you are using RPM) divided by the HP constant for one minute (33000, which is 550 Lb-Ft per second times 60 seconds per minute).

So- if the pressure is, say 1800 PSIG acting on a 4.00 inch piston with a stroke of 3.5 inches and there are 6 pistons inside a 4-stroke engine running at 3600 RPM, then the theoretical RAW horsepower is 1800 x (4.00x4.00/4x3.1416) x (3.5 inches / 12 inches per foot) x 3600 / 2 or about 2159 RAW HP.

That value must be reduced by friction factors of the walls of the cylinders andthe rings, pressure loss from the blow-by of the rings, friction of the connector rod bearing at the piston and the crankshaft and various other internal and external component friction factiors which can be identified by researching the engine design guides noted elesewhere but probably amount to at least 90%, leaving about 216 HP.

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#11
In reply to #10

Re: Calculating Engine Power Outputs

12/06/2009 8:16 AM

Hi guys

Thanks for all the comments.

Ive attached 2 screen captures.

1. is a cross section of the basic parts in the engine.

2. is a chart of the force in pound at the crank angles.

I know this is all theoretical, BUT it will be good practice for me to get some equations and using SI symbols for my course. This engine is nothing to do with my course this is just something I've been playing around with for a few months or so.

I am going to have a look at the books some of you recomended.

Gareth

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#12
In reply to #11

Re: Calculating Engine Power Outputs

12/06/2009 8:37 AM

On the spread sheet please ignore the first column as this was a different engine config.

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#13
In reply to #11

Re: Calculating Engine Power Outputs

12/06/2009 11:38 AM

I would just like to point out that Gareth still has not told us what type of engine he thinks he is designing. All of the responders ASS-U-ME that it is an Internal Combustion Engine [ICE] gasoline - but from the sketch - it could also be an External Combustion Engine [ECE]; e.g., Stirling. So getting back to the original question - what is the engine type? If an ICE - what is the fuel type; what octane rating; fuel air ratios, piston diameter, number of pistons, stroke length, etc., etc. before you even get to thermodynamics, frictional coefficients, metal + bearing types etc., etc.,

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#14
In reply to #13

Re: Calculating Engine Power Outputs

12/06/2009 12:08 PM

ATSTSUSA

The type of engine is nothing to do with my question. It is an internal combustion engine but not using fuel as in petrol or deisel. But that has nothing to do with it. The ingine will not produce pressure in the cilinder. may be some heat from the friction but thats it.

Back to my question. I have found a website that is taking me through the steps to find the theoreticle output of the engine power.

Also as you can tell by the spreadsheet the force it the piston top face at each degree of crank angle. So why do you need to know evey detail about the egines design?

Thanks again to everyone who has given good advice on material to read.

Gareth

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#15
In reply to #14

Re: Calculating Engine Power Outputs

12/06/2009 5:00 PM

"The type of engine is nothing to do with my question. It is an internal combustion engine but not using fuel as in petrol or deisel. But that has nothing to do with it. The ingine will not produce pressure in the cilinder. may be some heat from the friction but thats it."

See, here's your first situation. You WILL be developing pressure if in fact you are designing an internal combustion engine. That is a basic principle of IC engine design that you can glean from any thermodynamics text. That's how the engine works; force divided by area is pressure. You generate heat AND pressure from the combustion process.

Plus, ALL the design details influence the final horsepower output of the engine. All the details can improve or detract from the final output.

If you'll carefully review the history of engine design from the beginning of the industry to today, you'll see that a hundred+ years ago, huge engines (many liters or cubic inches of displacement) made 20, 30, 40 or so horsepower, even race engines.

Nowdays, F1 engines have made 900+ horsepower, and NASCAR engines can make 800+

Now, the thing is, these are all STILL (for the most part) 4 stroke naturally aspirated (no forced induction) internal engines.

Now, how do we get from 50 to 900+ horsepower, with with the same basic concept of a four stroke IC engine? Keep in mind, the pistons are still whizzing up and down, the crank and cam(s) are still spinning merrily along, and fuel is being combusted...

IT'S IN THE DETAILS...

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#16
In reply to #15

Re: Calculating Engine Power Outputs

12/06/2009 5:09 PM

I totally agree "If I was designing a standard engine" This is something totally new.

Going back to the question. It is simply this. can I calculate the power of an engine from the following factors, 1. the dimensions (as Ive drawn it up in Solidworks). 2. I only have the amount of FORCE in pounds pushing down on the pistons. Which is shown in the chart.

Gareth

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#17
In reply to #16

Re: Calculating Engine Power Outputs

12/06/2009 6:15 PM

Simply this, not really.

Horsepower is a function of torque and engine rpm. You can get a very rough idea of your output, but output is not an explicit function of the dimensions. The dimensions do play some role, but again, not explicitly.

Keep up your good work and keep thinking! I say this, because I am afraid you'll eventually discover that what you are proposing as "totally new" in fact has already most likely been proposed.

Every case that has been presented to this forum as "all new" usually receives (when the details come out) a response that it already exists, with references to scholarly documents, patents, professional papers, so forth.

Reciprocating piston engine design is a very mature technology. Breakthroughs in engine design will most likely be related to electronics and IT technology, and heat transfer/control.

Again, I do not say this in discouragement to your efforts. Just keep in mind that your idea may not be new.

To close, keep up the good work and have much success in your efforts.

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#18
In reply to #16

Re: Calculating Engine Power Outputs

12/07/2009 2:05 AM

In principle, yes, you can calculate the power at some operating rpm, given the engine geometry and the force on the piston, provided the forces are valid at that operating rpm. In any fluid-based engine (heated air, compressed air, hydraulic, etc) the pressures, and therefore forces, vary with speed. Even if the force were magnetic, (in repulsion, for instance) you'd wind up with eddy currents, the effect of which could change with rpm.

But if we set aside all the various and numerous disclaimers, then you can see that at 0 degrees, there is no rotation force on the crank caused by the piston force. At 90 degrees crank angle, the piston force is low but the leverage at the crank is better, so torque at that instant is higher. If you do the trig at each angle, you can find the torque at each angle, and then sum these increments and come up with an average torque through the power stroke. Some of this is lost to friction, so you would need to know more about the engine, but the friction losses might only be 10% or less. If there is a compression stroke, then the forces required would need to be accounted for in the same way. You could then get an average net torque, and with your selected rpm you could calculate hp.

You can see that at 90 degrees crank angle the piston is being forced against the side of the cylinder by the connecting rod angle. You could calculate this force, and figure the friction (based on materials, finishes and lubricants) and find out it my 10% guess is valid. (You'd use the same incremental process, calculating the friction for each crank angle and pressure).

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#19
In reply to #14

Re: Calculating Engine Power Outputs

12/07/2009 2:09 AM

The type of engine is nothing to do with my question. It is an internal combustion engine but not using fuel as in petrol or deisel.

Do you mean not using petrol or diesel, but using some other fuel. An internal combustion engine does not run without fuel, a requirement for combustion.

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#20
In reply to #14

Re: Calculating Engine Power Outputs

12/07/2009 9:06 AM

Gareth: You are wrong. No hypothetical dream machine will allow you to avoid the fundamental question of calculating the energy output from fuel combustion and relating that to the volume of fuel consumed in each cylinder. On the other hand you say that, "The ingine [sic] will not produce pressure in the cilinder [sic]". But you said it is an internal combustion engine. The statements are contradictory and mutually exclusive. If you do not understand this distinction I suggest that this discussion be terminated.

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#21
In reply to #11

Re: Calculating Engine Power Outputs

12/07/2009 10:31 AM

Now we are talking. In order to calculate the power output of this engine, all we need is 1) conversion of your lds units of force to common units 2) the stroke of the engine 3) whether this is a 2 cycle or 4 cycle engine 4) the rotational speed you want to make the calculation at. The calculation will assume the force on the piston is 0 between 180 and 360 degrees.

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#23

Re: Calculating Engine Power Outputs

09/21/2016 4:07 AM

Dear Mr. garth1422,

During 2nd year Engineering, the Internal Combustion Engines is a subjct and the Txt Book used was 'Theory of Machines, by the Author THOMAS BEVAN"

A formula is given for the power developed known as "Indicated Horse Power" and and the formula is

(P x L x A x N)/33,000 where

P = Mean Pressure in PSI, in side the Cylinder,

L = Length of the Stroke of Piston in Foot or Feet,

A = Area of the cylinder in Square Inches,

N = RPM of the Engine.

This IHP multiplied by Efficiency will give BHP i.e., Brake Horse Power.

DHAYANANDHAN.S

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