Performance comparison between jag and stock CDI

SchwinnStingray · Jul 17, 2014

Hoping to not look like I am singling you out here...

mew905 said:
Basically an engine (2 or 4 stroke) is powered by the expansion of gases, the conversion of gasoline and air into CO2 and other exhaust gases. Exhaust gases want to occupy considerably more room than oxygen and gasoline droplets (this is how Dynamite works too, it's not the flame that's deadly, it's the conversion of the materials into CO2 so rapidly that it produces a shockwave). This forces the piston downward. The faster you can make that flame burn through the chamber, the more power you will achieve. This can be done with more turbulence (better mixing), higher pressures (compression), ignition point (shortest distance, depends on if you run 1 or 2 plugs. 1 plug in the center for shortest distance to all sides), Air/fuel ratio (14.7:1 by weight burns the fastest, too much or too little will leave unburned oxygen or fuel), ...

Having a good background in V8 4 strokes I can tell you that there is alot of mis information in these paragraphs alone, let alone your general understanding of how the internal combustion engine makes power, not trying to have a go at you but if you are going to offer credible information for people to tear strips off one competitor or another it may as well be correct information.
The ideology you have is close but your interpretation is very misleading, so misleading I don't know where to begin...

Exhaust gases actually occupy less space (as they cool, as it is the HEAT that produces power, expanding on the piston) than unburnt inlet gases, this is why the exhaust valves are smaller than inlet valves in a 4 stroke.

It is actually a slower burn that will make more power, the higher the compression, the more advanced the spark the more we have to slow and cool the burn to prevent detonation.

14.7:1 is the ratio where 90 to 100 octane pump fuel burns the cleanest, the hottest burn will be much leaner than this.

FWIW I still think that the only good thing these black boxes have to offer is separating the CDI from the coil, just like about every other cycle manufacture out there...
A high end ignition system where tailored advance curves, like what they use in the kart racing fraternity would show improvements where needed.
I'm sorry I don't know enough about the internal magic of a two stroke as yet but I am learning, as far as I have seen there is very little difference in the actual needs of the 2 stroke compared to the 4 stroke, event timing, ignition timing, mixture and spark...

mew905 · Jul 17, 2014

From an engineer:

Several assumptions: 1) This is a gas engine we are talking about, 2) it is a four-cycle gas engine, 3) Combustion will be stochiometric and complete, 4) the compression ratio is about 10:1, 5) the engine is throttled (no variable valve timing), 6) normal aspirated engine (no turbocharger), and 7) volumetric efficiency (the amount of air that makes it into the cylinder during the induction stroke) is 1.00 (actually it depends upon the RPM and intake manifold pressure, but work with me here.)

First, it should be intuitively obvious to the most casual observer that the amount of air that passes through the engine in will be equal to the engine displacement times the RPM divided by 2. For an engine of 3 liter displacement going at 3000 RPM, the amount of air pumped for minute will be 4500 liters.

That will approximately be the intake volume flow for an engine with the throttle wide open. If we assume that the throttle is only open 33%, the intake volume flow will still be 4500 liters, but the pressure will be one-third of an atmosphere. The equivalent mass of air will be the same as 1500 liters at one atm of pressure.

Neglecting the addition of the fuel mass, the mass of the exhaust gas will be the same as the mass of the intake gas. From the ideal gas law we know that the increase in volume of the exhaust gas will be proportional to the increase in absolute temperature. If we assume an intake temperature of 80 deg F, and an exhaust temperature of 1800 deg F (reasonable assumption, depends upon compression ratio), the absolute temperature will be 540 and 2260 deg Rankine, respectively. The volume increase will therefore be 2260/540, or 4.185.

For the hypothetical 3 liter engine running at 3000 RPM and full throttle, the exhaust gas volume will be about 4500*4.185, or 18,833 liters/min. At one third throttle the corresponding flow is 6277 liters/min. Since one cubic foot is equal to 28.3 liters, the respective CFM flows will be 665.4 and 221.8, respectively.

How about the contribution from combustion products? Assuming stoichometric combustion, there will be one pound of fuel burned for each 14.55 lbs of air. Air is 21% oxygen, so there is 3.05 lbs of oxygen available to burn each pound of gas.

A reasonable chemical approximation for gasoline is octane, which has a chemical of C8H18. The molecular weight is (12*8+18*1)= 114.

The combustion formula is C8H18 + 12.5 O2 ==> 8 CO2 + 9 H20. For each 114 grams of C8H18, there will be 12.5 moles of oxygen consumed, producing 8 moles of CO2 and 9 moles of H2O. For gas volume purposes, since equal moles of gas produce equal volume, the volume of exhaust gas replacing oxygen will be equal to 17/12.5 = 1.36.

The volume percentage of oxygen in air is about 21% (not exact, but work with me here). This volume will be removed, and replaced by exhaust gas with a "volume" of (21*1.36) = 28.56%. The resulting post combustion volume is (79% + 28.56% = 107.56%) of the pre- combustion volume -- assuming no temperature increase.

So what do we have? Combining the increase in volume from combustion reactions and thermal expansion, an engine with a 3 liter displacement running at 3000 rpm with the throttle wide open will have an exhaust volume (at 1800 deg F) of 665.4*1.0756 ~~ 715 cubic feet per minute. For the throttle one-third open, the exhaust flow will be 238.6 cfm.

In short, exhaust gases do consume more space than air/fuel mix, and because of the drastic temperature increase in exhaust gases, they take up even more space. You're partially right in saying it's the heat that creates the power but it is minimal, as it simply decreases density of exhaust gases, making them want to take more space. In fact, part of the reason gasoline engines are inefficient is because a lot of the work of the engine is transmitted into heat. In fact any time you produce heat, you're losing efficiency, unless of course creating heat is your goal (such as an oven burner). The engine, however, is not designed to make heat, its designed to make power, there's a reason we cool them, not heat them. I'm not familiar with the workings of a 4-stroke or the shapes of the valves but if the exhaust ports are smaller, it sounds like it's done for a purpose other than exhaust gases being less dense. According to that post, the exhaust gases, at the same temperature as air-fuel mix, will consume twice the space. But because of the heat produced in the process, it consumes much, much more.

EDIT: Apparently exhaust ports on 4 strokes are smaller because the exhaust gases flow faster. As exhaust gases are forced out by the piston, the intake gases are inducted by atmospheric pressure, requiring a larger port on the intake to allow air in less restricted.

SchwinnStingray · Jul 17, 2014

The limiting factor of the internal combustion is it's ability to handle heat, more gas in, the bigger the bang, the more heat.
In a water cooled 4 stroke approx 1/3rd of heat is lost out the exhaust, 1/3 radiated and 1/3 being the working energy.
Change any one of these factors in your favor will add power.

If you could make it go bang without the heat then sure power would rise but the heat is energy and alot of wasted energy it is.
This is why cooler burning alcohol produces more power at a higher compression ratio.

I'm sure an air cooled 2 stroke would have around a 50/50 loss.

mew905 · Jul 17, 2014

SchwinnStingray said:
The limiting factor of the internal combustion is it's ability to handle heat, more gas in, the bigger the bang, the more heat.
In a water cooled 4 stroke approx 1/3rd of heat is lost out the exhaust, 1/3 radiated and 1/3 being the working energy.
Change any one of these factors in your favor will add power.

If you could make it go bang without the heat then sure power would rise but the heat is energy and alot of wasted energy it is.
This is why cooler burning alcohol produces more power at a higher compression ratio.

I'm sure an air cooled 2 stroke would have around a 50/50 loss.

Yep, it's a sad fact of physics but if it weren't for that, we'd have perpetual motion machines and unlimited energy. I'm not saying that wouldn't be nice though, but you're right, I could never really understand why, all factors equal, that a water cooled engine produced more power than a gasoline engine, but it's that heat dissipation, allowing more energy to go into producing power. Physics is weird, but I like to know why things do what they do on the smallest relevant level. However I'm not sure if cooling the exhaust gases would have a good impact on performance, from what I understand, it's detrimental. 2 strokes are far more complicated (physics wise) than 4 strokes are. Definitely much easier to think about it in a 4-stage sealed chamber, than a "piston goes down and simulaneously releases exhaust gases and pushes a new intake charge into the cylinder".

SchwinnStingray · Jul 17, 2014

The ideal would be to retain all the heat in the combustion process as working energy, something they are experimenting with ceramic engines.

as a side note on cooling exhausts, cooler gas is slower gas as it cools it contracts, the slower the charge exhausts from the engine the more power it will rob as it is like back pressure, polluting the intake charge in extreme cases.
Again heat wrap and ceramic coatings help here.

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Performance comparison between jag and stock CDI

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