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Old 03-02-2014, 12:10 PM   #71 (permalink)
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Thanks Old Tele man.

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Originally Posted by Old Tele man View Post
Like this (1L = 61.024 cid) quick answer:

T = (hp 5,252)/(rpm)
T = (12hp 5,252)/(2,000 rpm) = 31.512 lbft

BMEP = 150.8 (T/D)
BMEP = 150.8 (31.512 / 61.024) = 77.871 psi

However, more specific engine data is needed for meaningful results--like bore & stroke, CR, VE, value timings--for determining piston velocities, dwell times, assumed intake/exhaust flow rates, etc.
The quick answer is adequate.

With conversion into liter displacement input, the formula is simply:

BMEP = ( HP * 13000 ) / ( L * RPM ) , for a four cycle engine.

But the result is the same. 78.1 psi .

For those not familiar with BMEP, it is a quantity that represents an ideal, flat pressure head acting on the piston top across the angular range between TDC and BDC on the power stroke ( 180 degrees piston motion ). It's usefulness is in comparing ICE engines across a broad spectrum of design variables. It is usually introduced in undergrad engineering classes as a derivation exercise that is surprising in that the resulting pressure is dependent only on the variables of displacement, engine speed and power - three values that are easily found in literature for a particular engine, or measured in the college dynamometer lab.

We can use this equation to glean some insight into engine modifications.

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Old 03-02-2014, 02:17 PM   #72 (permalink)
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The first thing that jumps out at you . . .

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Originally Posted by RustyLugNut View Post
The quick answer is adequate.

With conversion into liter displacement input, the formula is simply:

BMEP = ( HP * 13000 ) / ( L * RPM ) , for a four cycle engine.

But the result is the same. 78.1 psi .

For those not familiar with BMEP, it is a quantity that represents an ideal, flat pressure head acting on the piston top across the angular range between TDC and BDC on the power stroke ( 180 degrees piston motion ). It's usefulness is in comparing ICE engines across a broad spectrum of design variables. It is usually introduced in undergrad engineering classes as a derivation exercise that is surprising in that the resulting pressure is dependent only on the variables of displacement, engine speed and power - three values that are easily found in literature for a particular engine, or measured in the college dynamometer lab.

We can use this equation to glean some insight into engine modifications.
. . . just how relatively low the pressure is in this idealized example. This is because it is idealized ( mean = average of the pressures ) not an actual pressure trace. An actual pressure trace would have peaks that are much higher and plateaus that are much lower. It is also a NET idealized pressure, taking into account the losses of pressure before TDC and after BDC.

Also, it is representative of the power output we as ecommoders strive to reach in our efforts to maximize fuel consumption. Many of the Xprize cars like the Edison VLC, the Aptera and Jack McCornack's Max, need less power than this to cruise at road speeds. But it is certainly a bottom number many of us can strive for with some of our modified cars.
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Old 03-02-2014, 03:42 PM   #73 (permalink)
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Perfomance engine tuners . . .

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Originally Posted by RustyLugNut View Post
. . . just how relatively low the pressure is in this idealized example. This is because it is idealized ( mean = average of the pressures ) not an actual pressure trace. An actual pressure trace would have peaks that are much higher and plateaus that are much lower. It is also a NET idealized pressure, taking into account the losses of pressure before TDC and after BDC.

Also, it is representative of the power output we as ecommoders strive to reach in our efforts to maximize fuel consumption. Many of the Xprize cars like the Edison VLC, the Aptera and Jack McCornack's Max, need less power than this to cruise at road speeds. But it is certainly a bottom number many of us can strive for with some of our modified cars.
. . . will look at the BMEP equation and realize that any modifications that maximize BMEP will yield more power at the given rpm. The straightforward solutions of more air, more fuel and optimized ignition timing lead to supercharging, higher flow fuel systems and advanced ignition systems with knock sensor feedback. These and other performance modifications certainly result in increased effective cylinder pressure, but which ones are relevant to the goal of the Ecomodder? We do not want to drive around at nearly full throttle all the time, but as many of us know, this is the throttle setting for a spark ignited gasoline engine that results in the best power output for the lowest fuel input or the lowest BSFC ( Brake-specific-fuel-consumption ).

Since we cannot drive around at heavy load, we pulse and glide to get an "average fuel use" that is considerably less than a partially throttled engine driving the same distance. Engine designers have used cylinder de-activation to achieve the same goal but with varying degrees of success.
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Old 03-02-2014, 03:55 PM   #74 (permalink)
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Quote:
Originally Posted by RustyLugNut View Post
The quick answer is adequate.

For those not familiar with BMEP
Add on emphasis:

As noted previously the method used above is an idealization , that makes certain assumptions. Real world operating engines may not conform to the numbers calculated from this or other similar types of calculations. Usually real world engines are almost always +/- some degree of variation away from these kinds of generalized/idealized calculations... Which is why properly performed and controlled real world testing always trumps the 'simulation' / 'calculation'... Although if we remove enough of the 'idealized' and enough of the 'assumptions' it is possible to get far more accurate 'simulations' of what will happen in the real world ... This usually makes things much more complicated ... thus the value of computers to run those more complex simulations ... or for the initial rough draft use of these kinds of idealized/generalized calculation methods.

- - - - -

ok ... so we start with this quickie estimate.
Now ... by all means please proceed.
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Old 03-02-2014, 04:05 PM   #75 (permalink)
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. . . will look at the BMEP equation and realize that any modifications that maximize BMEP will yield more power at the given rpm.
I feel it is important to note:
#1> Significant difference between power and energy.
#2> More Power does not necessarily mean more energy.

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Originally Posted by RustyLugNut View Post
These and other performance modifications certainly result in increased effective cylinder pressure, but which ones are relevant to the goal of the Ecomodder?
Only those that increase the net energy efficiency.

Quote:
Originally Posted by RustyLugNut View Post
We do not want to drive around at nearly full throttle all the time, but as many of us know, this is the throttle setting for a spark ignited gasoline engine that results in the best power output for the lowest fuel input or the lowest BSFC ( Brake-specific-fuel-consumption ).
A gross over simplification ... not always correct ... but , will let it go at that for now.

- - - - - -

ok ... go on.
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Old 03-02-2014, 04:31 PM   #76 (permalink)
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Rusty enjoying this thread.

Thanks!!!
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Old 03-02-2014, 04:42 PM   #77 (permalink)
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Quote:
Originally Posted by RustyLugNut View Post
We do not want to drive around at nearly full throttle all the time, but as many of us know, this is the throttle setting for a spark ignited gasoline engine that results in the best power output for the lowest fuel input or the lowest BSFC ( Brake-specific-fuel-consumption ).
And why would that be? Maybe it has something to do with the fact that there's no intake manifold vacuum to consume work (therefore, gasoline)?

Besides, speak for yourself. I would love to be able to drive around at full throttle, for the very reason that I would not have to deal with any sort of intake manifold vacuum. The Karen-Mobile already opens up its EGR valve farther than spec, to approach this exact goal. Going to modify the Caravan, for the same reason, once I get an MPGuino installed into it. Later on this spring, I'm going to install a 2.87 rear end in the Karen-Mobile to lower intake manifold vacuum (it currently has a 3.636 rear end). I'm also probably going to experiment with WBO2-based EFIE on the Caravan to get it to do a version of lean burn, also for this purpose.

I still cannot see that your fancy Browns Gas generator (or whatever you want to call it) is going to do anything out of the ordinary, that more pedestrian engine mods won't already do. But, by all means, let's see what sort of magical gains can be had. Just don't ask me to pony up any money for your hucksterish claims.
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Old 03-02-2014, 04:45 PM   #78 (permalink)
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This line of thinking . . .

Quote:
Originally Posted by RustyLugNut View Post
. . . will look at the BMEP equation and realize that any modifications that maximize BMEP will yield more power at the given rpm. The straightforward solutions of more air, more fuel and optimized ignition timing lead to supercharging, higher flow fuel systems and advanced ignition systems with knock sensor feedback. These and other performance modifications certainly result in increased effective cylinder pressure, but which ones are relevant to the goal of the Ecomodder? We do not want to drive around at nearly full throttle all the time, but as many of us know, this is the throttle setting for a spark ignited gasoline engine that results in the best power output for the lowest fuel input or the lowest BSFC ( Brake-specific-fuel-consumption ).

Since we cannot drive around at heavy load, we pulse and glide to get an "average fuel use" that is considerably less than a partially throttled engine driving the same distance. Engine designers have used cylinder de-activation to achieve the same goal but with varying degrees of success.
. . . was one that defined the efforts of the three aforementioned Xprize teams. Each of them responded with different solutions.

The Edison team designed a custom engine that could produce power with a wide valley of low BSFC and also leveraged the generous Xprize rules for specific energy content for fuel volume. Building around the high octane inherent in the E85 fuel, their engine propelled their sleek and light weight car to the class win.

Jack McCornack looked at the requirements for hitting 100 MPGe (MPGe was the Xprize way of equalizing all the fuels and their differing make ups ) and decided on the expedient solution of using a current engine that already had low BSFC over a wide range because of no throttling losses - he chose a turbo diesel engine to power his entry car, "Max". Even with the higher diesel energy factor requiring 114 MPGe, his "napkin calculations", coupled with a sleeker fiberglass body and idealized gearing would have allowed Jack to be competitive if he had not been forced to withdraw due to "production" rule changes.

Aptera's offices and labs were only about half an hours drive from my home in San Diego. I considered them one of the front runners in the 2 seater class before technical difficulties not directly due to their brilliant design left them out of contention. They took the simple expedience of not dealing with engine design or selection and simply went with the most efficient drive train available to them - they used an electric/battery drive.

The disappointing part of the Xprize and the after results is the lack of immediate technology transfer to the everyday world. I had hoped an ICE evolution that would carry forward the momentum of engine tech such as the stratified charge engines from Mitsubishi in the 70s, the lean burn engines of the 80s, and the effective Honda Lean Burn VX engines that came after would make an appearance in the competition. Engine technology would transfer the most readily because consumer acceptance of what is under the hood is very broad - if it works, most people really don't care what powers their car. Unfortunately, engine and drive train development is difficult in comparison to light weight and aerodynamic body design and construction. The backyard tinkerer or the small development group would be hard pressed to compete with the large corporations in the time frame provided. And, no large corporate groups committed to compete.

So where does that leave the average Ecomodder and his equation for BMEP?

Last edited by RustyLugNut; 03-02-2014 at 05:45 PM.. Reason: Spelling.
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Old 03-02-2014, 06:25 PM   #79 (permalink)
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The very interesting thing about BMEP is what variables change it and which ones don't.
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Old 03-02-2014, 07:19 PM   #80 (permalink)
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The very interesting thing about BMEP is what variables change it and which ones don't.
...as well as the proportional changes netted upon BMEP, ie: some effects are micro and others macro.

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