3rd Compression Ratio

[wobblywalrus]

Right now I am serving on a grand jury.  There is lots of down time between cases and I need something, anything, to read.  So I goes to the used bookstore nearby and buys "Secrets of Model T Speed:  the Fast Ford Handbook" by Murray Fahnestock.  He is writing in 1917 about racing Fords from Ohio.  He says:  "It was very seldom that any special effort was made to increase the compression by planing off the cylinder head.  When the valves are enlarged, or the timing changed, by using a different camshaft, or when overhead valves are used, the extra gas, admitted to the cylinders, automatically raises the compression ratio about as much as can be used to advantage.  When more gas is compressed, in the same space as before, the compression must naturally be higher."

This is something I have noticed throughout the years.  There is the static compression ratio, the dynamic compression ratio, and another compression ratio for the engine running with peak volumetric efficiency.  Does anyone know how to calculate this 3rd compression ratio?       

[4-barrel Mike]

Be careful of that book, Bo!  You'll end up building a V4F to put in that roadster of yours and planning how to set records with it.

 

Mike

[Dean Los Angeles]

Static compression ratio really doesn't exist in any meaningful way. The volume ratio between bdc and tdc is an easy talking point, but nothing else.

There is no such thing as "dynamic" compression ratio. Because someone used the word "static" then "dynamic" is the correlated word, but it means nothing here.

Dynamic conditions change with operation, and the dynamic compression ratio never changes. Better to call it the "calculated" or "corrected" ratio. Always lower than static compression on a race engine because of the late intake valve closure.

What you would refer to as the 3rd compression ratio is actually the cylinder pressure. And it is very dynamic.

The cylinder pressure developed when an engine is running will be higher than that shown in a compression test for several reasons.

The much higher velocity of a piston when an engine is running versus cranking allows less time for pressure to bleed past the piston rings into the crankcase.

A running engine is coating the cylinder walls with much more oil than an engine that is being cranked at low RPM, which helps the seal.

The higher temperature of the cylinder will create higher pressures when running vs. a static test, even a test performed with the engine near operating temperature.

A running engine does not stop taking air & fuel into the cylinder when the piston reaches BDC; The mixture that is rushing into the cylinder during the downstroke develops momentum and continues briefly after the vacuum ceases (in the same respect that rapidly opening a door will create a draft that continues after movement of the door ceases). This is called scavenging. Intake tuning, cylinder head design, valve timing and exhaust tuning determine how effectively an engine scavenges. Ram air changes the equation.

Supercharging REALLY changes the equation.

Compression ratio is the ratio between two volumes. The pressure ratio is the difference in pressure between the two volumes.
The pressure ratio on a 10:1 compression engine is 22:1. The pressure ratio on a 15:1 compression engine is 40:1.

[wobblywalrus]

Dean, what I think is being said is the cylinder pressure goes up when the volumetric efficiency increases, even if the compression ratios do not change.  Is there a chart or something that correlates the octane needed in a gasoline to cylinder pressure?

[Jon]

There is no direct correlation between compression ratio and octane requirements.
A long flame front travel distance will make a cylinder more prone to detonation than cylinder head with a short flame front travel distance.
One of the reasons a late model bike engine with a central plug can run higher compression with the same fuel than a big block with an open chamber and plug off to one side.

Lots of other things contribute as well.

jon

[wobblywalrus]

It seems I need to do the changes that affect how much mixture gets into and out of the combustion chamber first and then optimize fuel specific gravity, octane, and timing.

The reason are I am going to use pointier cams and a more free flowing exhaust.  More mixture will go into the cylinders to get compressed at high engine speeds.  This raises the combustion pressure.

Mike, my shed is pretty small and the only car that would fit would be a little belly tank or other short and open wheeled, narrow bodied car.  The speedster things they are racing in this Model T book fit the bill.           

[Dean Los Angeles]

There isn't a formula, but I'm sure there is a multimillion dollar super computer that would do the job!

The holy grail is:
Push as much air into the engine as possible.
Determine exactly how much air that is.
Add fuel to achieve a stochiometric ratio of 14.7:1.
Compress it as much as possible.
Ignite it.

The vast bulk of engine work is putting air into the engine. All NA engines are ultimately air limited. That includes getting the exhaust out so that it doesn't pollute the next cycle.
If you are running EFI you can use a mass air flow sensor to determine how much air is going in. Don't forget that is constantly changing with altitude, temperature and humidity. Don't have that? Then dyno time and experimentation is how you find out. Changing the jets in the carburetor until you find the right setup for that altitude, temperature and humidity gives you an index to change when you are competing. (Not REAL sure why anyone would run a carburetor . . .)

If you were to add fuel at exactly 14.7:1 AND completely combine it so that EVERY fuel molecule had a matching air molecule you would reach the highest heat value (and HP) possible.

It isn't possible. In the hundredths of a second that this all happens, it isn't possible to mate every molecule. That's why you hear ratios closer to 12:1 achieving peak power on the dyno. That's a testament to the failure to mix everything. On an EFI system the use of a wide band air fuel ratio sensor in the exhaust will tell you (and adjust) the air/fuel ratio.

If you are running a carburetor the fuel is pushed into the air stream by the difference in pressure between the float bowl and the venturi. If you look at it while it is running it looks like a nice spray. It isn't. Those massive drops of fuel have no chance of igniting. The turbulent air flow and heating through the hot engine and compression somewhat vaporize and mix it before ignition. Somewhat.

Every car sold has fuel injection. Even NASCAR finally went to fuel injection. One of the reasons is the ECU controls the system, not a bunch of fuel passages in the carburetor. (Have you ever rebuilt a Holley carb?) The other is that fuel injection takes advantage of the high fuel pressure. The typical system runs 40-60 psi. When the fuel is released by the injector the sudden drop in pressure, and the design of the nozzle breaks the fuel into micro small drops. Higher and higher pressures are coming to fuel injection to more completely atomize the fuel. Some diesel injectors are running 26,000 psi!

The knock sensor on your EFI is the answer to octane and detonation. Detonation (knock, ping, etc.) happens at the wrong time in the cycle and isn't what you want. That is always the limit on high compression. High octane numbers are only a partial cure. You can't predict the complete cycle of rpm, air changes, engine loading, etc.

The bypass to calculating this (or buying that super computer) is to spend dyno time to learn how your particular engine reacts.

[WOODY@DDLLC]

Dr Dean Hill speaking at the 16th AETC event at PRI: [Paraphrased by me!] "Only 0.05% of available air/fuel molecules will react in the 60,000°F spark of .001 second duration. So how can an engine even run? A 350 cid V-8 with 12:1 compression running at 6000 rpm has a billion billion molecules each with an average speed of 2500 mph passing through the spark sphere 10.8 times!" Even the super computers struggle with that!  So all you tuner guys are doing pretty durn good! 

[wobblywalrus]

The children and coming by for trick-or-treat dressed like little cats, bananas, etc.  They sure are cute.

This engine is pretty efficient.  The blue dyno curve is the best main jet size.  The needle was shifted to get the red curve and it is the final 2013 setting.  The mixture does not need to be overly rich to make the best power.  My figuring is the atomization is pretty good.  The carbs do not have the obstruction from throttle plates in the venturis, they have only little needles, so the carbs look like the best power producing option for now.

The displacement and static compression ratios for the three engines I have built did not change during their lives.  The improvements were in getting more mixture into the cylinders.  The BMEPs calculated by PipeMax increase during the engine lives as I made improvements.  Is the BMEP is sort of like the third compression ratio I am looking for?       

The changes in BMEP through the engine lives are not shown.  This is some data from PipeMax for the three different engines:

The final 790cc build:  VE = 108%, static comp ratio = 9.2:1, BMEP = 183.8, octane requirement = 91.0 to 91.8
The final 865cc motor:  VE = 109%, static compression ratio = 10.5 :1, BMEP = 194.6, octane requirement = 95.8 to 96.5
The 995cc build in 2013:  VE = 105%, static compression ratio = 10.5:1, BMEP = 181.5, octane requirement = 95.5 to 96.5 

 

[Interested Observer]

Wobbly,
Based on the power curve of your reply #8 and assuming that is  for the 865 cc engine, the actual achieved BMEP’s are:
3000   165.3 psi
4000   180.3
5000   183.4
6000   187.9
7000   178.2
8000   155.0

(MEP = 13E6*HP/RPM/cc)

How is the air/fuel ratio measured on this dyno?  Has it ever been calibrated?  It seems to consistently indicate rather more on the lean side than one would expect.

[wobblywalrus]

It is a 995 cc engine.  The O2 sensor is stuck up the end of the exhaust pipe.  I asked about the sensor.  It is believed to be accurate.  Gasolines have different stoich ratios and the air/fuel ratio for Sunuco Standard is 15 to 1.  That is leaner than the measured mixture.

The bike has always run good on ERC MULB and this is an oxygenated unleaded.  The BMEP's for the previous engines that used it are higher than the BMEP for the current motor.  This makes me think I can run an oxygenated fuel like ERC MULB without ill effect.  I would use it, except the fuel is hard to get here in Oregon and I had no luck in finding a source in South Australia.  The Sunuco is easier to find.

[wobblywalrus]

Thanks, Interested Observer.  Some calculations on the engines show the peak BMEP occurs during the torque peak.  This makes sense and it explains a lot.  We tuned and rode British bikes during the early 70's and raced the Japanese scoots.  Ignition timing and compression were maximized.  There were times we had knock at the torque peak and it would not go away with reasonably richer mixtures or retarded timing.  Lowering the compression or using avgas were the only solutions.   

[Interested Observer]

Wobbly,
The BMEP curve is directly proportional to the torque curve.  As you can note from the equation in reply #9, horsepower divided by rpm is torque.  The BMEP is just a useful figure of merit to use in comparing output of various engines and is a window into their respective volumetric efficiencies.

[Dean Los Angeles]

The dyno works by placing a load on the engine, or "brake".

Mean Effective Pressure derived from a dyno is Brake Mean Effective Pressure (BMEP).

The definition of BMEP is: the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output. Note that BMEP is purely theoretical and has nothing to do with actual cylinder pressures. It is simply a tool to evaluate the efficiency of a given engine  producing torque from a given displacement.

Maximum Torque and BMEP occur at the same place. This is where the engine is working at best flow efficiency. BMEP allows you to compare to a vastly different engine and see where you line up. A blown fuel motor can be as high as 1000 psi. 180 psi for a normally aspirated engine is pretty good.

On the other hand, 140 psi is possible on a piston ported two-stroke. No valves, cams, etc. And twice the number of power pulses as a 4-stroke. No wonder they are popular.

As the rpm increases the flow efficiency starts to drop off, but the number of power pulses more than makes up for it, and the HP numbers go up. Peak horsepower is when the two curves cross and the drop in flow efficiency isn't matched by the increase in power pulses. That doesn't mean you can't go faster with more rpm's, but it really is time to shift gears.

[tortoise]

So I goes to the used bookstore nearby and buys "Secrets of Model T Speed:  the Fast Ford Handbook" by Murray Fahnestock.  He is writing in 1917 about racing Fords from Ohio.  He says:  "It was very seldom that any special effort was made to increase the compression by planing off the cylinder head.     

On an L-head engine, milling the head reduces the flow between the valves and the cylinder, so Mr. Fahnestock was right. These newfangled ohv motors don't have that problem.