Internal combustion expansion / cylinder pressure?

[Bob Farr]

I'm trying to improve my basic understanding of cam timing and cylinder pressure (BMEP?) and I'm having a bit of difficulty getting my mind wrapped around it. I know that steam engines rely in part on a predictable rate of steam expansion. I'm curious whether an air/fuel mixture has a predictable expansion rate which could be explained in simple terms.

I'm sure that the mix ratio, effective compression ratio, temperature, humidity and other variables have an impact, but is there a baseline principle which can be applied to predict how much "volume X" of a mixture of "y ratio" will expand/displace when ignited, and how fast?

My application is the cam timing, ignition timing and intake manifold volume of a v-twin four-cycle engine (basic 45-degree Harley layout).

Thanks in advance for any tips,

Bob

 

[mu38&Bg#]

The pressure plot of an IC engine is dependent on fuel characteristics, air/fuel ratio, ignition timing itself, mixing of air and fuel, compression ratio, and turbulence in the combustion chamber. This fills a good part of any engine design text. This kind of stuff is computer modeled today. BMEP is dependent on everything combined, but mostly on cylinder scavenging and compression ratio. Cylinder scavenging is much more than just valve timing. You have to get into port shapes, runner lengths of both intake and exhaust.

Will the engine be used to pull a load?

 

[Bob Farr]

*** Will the engine be used to pull a load?

Eventually yes, but I'm thinking of modeling some ideas first.

The full-scale machine is my 1974 FLH, so we're talking about a gas engine (CV carb), roughly 14:1 mix ratio, currently an 8.0:1 static compression ratio, single spark plug, hemi-head engine (no squish band). The intake tract is very short on this engine, and I've been playing with cams of different overlap and duration to manipulate the effective compression ratio, intake ram effect, exhaust column vacuum effect, etc. I've had some success in improving performance (both power and economy).

The model engine idea comes from a 1953 Harley Hummer which is available near me, but without an engine. I have a set of 1967 FLH (generator style) engine cases to copy from, and a 1/2 or 2/3-scale model v-twin could be wedged into the Hummer frame to make a neat little bike in the 350-500cc range. Building such an engine from scratch leaves a lot of opportunity for interpretation and modification: longer intake tracts, modified head layout, etc. But I need a better grasp of the principles.

I have a nice copy of Helt, High Speed Combustion Engines, NYACK, N.Y. 1944. It helps, but simple explanations are good too

 

[Lakc]

I have a nice copy of Helt, High Speed Combustion Engines, NYACK, N.Y. 1944. It helps, but simple explanations are good too

The modern equivalent of that book is called:
The Internal-combustion Engine in Theory and Practice Vol, I&II By Charles Fayette Taylor

Two volumes make it a bit pricey, but its an excellent read, even if the math goes over my head most of the time.

 

[gbritnell]

Hi Bob,
Here's a quick and dirty explanation of BMEP.
http://www.harleyc.com/prelude/articles/AllAboutMeanEffectivePressure/default.html
A little searching on the web will produce all kinds of scientific examples.
To make a small scale engine the best advice I can give you is use moderate numbers for all your factors, comp. ratio, no more than 8:1, full ignition timing, no more than 30 degrees, and as for cam timing I would just use moderate overlaps.
If you start using radical numbers for a miniature engine you're going to have to make compensations for cooling, stress, material strengths and vibration to name a few.
gbritnell

 

[mu38&Bg#]

Mr Taylor's books contain a lot of information. Gordon Blair's books are the modern reference, but discuss mostly gas flow and computer modeling. Another book to have is "Internal Combustion Engine Fundamentals" by John B. Heywood.

350-500CC is a serious engine. Even conservative valve timings can make good power if breathing is good. Engine power is mostly about breathing.

Google will turn up some helpful information. The link below contains some scans from the Heywood book.

http://forums.evans-tuning.com/viewtopic.php?f=3&t=136

 

[Admiral_dk]

Bob are you trying to get as high a power output as possible or are you asking simply for data to calculate material strength ?

Personally I would start with looking at an engine of the same size and power to get a good idea of the dimensions, though I'm aware of the problem with obtaining the same materials / same treatment to get the same strength in a few areas - Pistons, conrods, crank and transmission. These areas might be a problem if you want high power (admittedly not something I associate with a Harley)

I would love to build a motorcycle engine, but I'm sure I would get some quality pistons elsewhere and if my power ambitions climb over a certain level I wouldn't make the crank myself / alternatively have a specialist do the final balancing, hardening and grinding.

 

[1Kenny]

Hi Bob,

Here is the NASA technical site. The link posted is the NACA portion. It lists all the studies done on internal combustion engines. I searched for internal combustion pressures and it came up with 84 reports. You could also try internal combustion temperatures. There is no simple answer to you question due to the variables but they will give you a good idea.

http://ntrs.nasa.gov/search.jsp?Ns=...ode matchall&Ntt=internal combustion pressure

Kenny

 

[Bob Farr]

Thanks for the tips and the research links gents. I have a lot of studying to do.

Bob

 

[Bob Farr]

Some study has revealed that the expansion ratio of an ignited gas/air mixture is approx 8:1, with diesel mixtures much higher at about 17:1. I'm not quite clear yet whether that 8:1 expansion ratio related to the total physical volume of the entire cylinder (i.e.,125cc in a 125cc motor) or the head volume remaining for the compressed mixture at either TDC (or possibly the ignition point volume a few crank degrees before before TDC). My guess is 8:1 over the compressed head volume at TDC will give me the expanded volume during combustion. Any thoughts?

*** This kind of stuff is computer modeled today. BMEP is dependent on everything combined ***

Dieselpilot, is there some simple freeware modeling software available which will allow me to alter intake volumes, compressed head volumes, compression ratios, exhaust volumes, etc to help narrow down the range of functional combinations?

Thanks,

Bob

 

[Lakc]

Some study has revealed that the expansion ratio of an ignited gas/air mixture is approx 8:1, with diesel mixtures much higher at about 17:1.

Despite the similarity to compression ratios (or perhaps because of it?) that ballparks around what I have heard for modern passenger car data. 8*14.7=117.6. I have heard in the 120-160 psi range for standard gasoline passenger car engines. Diesel's will obviously be higher.

Dieselpilot, is there some simple freeware modeling software available which will allow me to alter intake volumes, compressed head volumes, compression ratios, exhaust volumes, etc to help narrow down the range of functional combinations?

There was, at one time, a crippled demo version of some hot rod shop's engine modelling software. It was very detailed in the amount of data to input, covering a great number of variables in camshaft and cylinder head design. While fun to play with, it wasnt aimed at model engines, some inputs could not be small enough as I recall. For a whole bunch of reasons regarding things like laminar flow, navier-stokes, and a bunch of other things I have read but not fully understood, I dont expect any of those programs to "scale well" down to our size.

Ha, would you believe it, I actually still had the link saved: its called engine analyzer pro.
Caveat, I have not been here in at least 7 years, several lifetimes in 'net sites.
http://performancetrends.com/download.htm#eapro

 

[mu38&Bg#]

Compression ratio is simply cylinder volume at BDC/ volume at TDC.

Real compression ratios are not what they look like on paper. The intake valve closes ABDC in most engines and volumetric efficiency can vary from 60% to over 100%. Hot racing engines may open the exhaust valve 90-100° ATDC to clear the cylinder for the next cycle. Making power is not about expansion ratios, that's mostly fuel conversion efficiency. Volumetric efficiency makes power. There is much more in the details of the flow path than there is in the actual valve timing when you can't do acoustic/dynamic flow tuning of the manifolds. All modern (as of 1993 or so) engines have tuned intake runners. many of this years production vehicles have compression ratios over 10:1, some up to 12:1.

I've downloaded Lapsim from Bosch, but haven't tried it yet so I have no idea what it's capable of. http://www.bosch-motorsport.de/content/language2/html/3589.htm

 

[Kermit]

It is very well established that the explosion, and therefore the heat created, causes the gases in the combustion chamber to obey standard rules of Chemistry, such as the Ideal Gas Law. Because of the sudden heat, the gases try to expand immediately, but they cannot, so the pressure in those hot gases greatly and rapidly increases. The explosion pressure in an internal combustion engine rises to between 3.5 and 5 times the compression pressure, and peaks at around 500 PSIA. Near the bottom of the power stroke this drops to 100-150 PSIA.

The flame front or propagation wave of the burning fuel moves at 90ft/sec(about 60mph) and will travel 4 inches in approx. .003 sec.

Kermit