Into the weeds engine design for fuel efficiency discussion.
The difference between efficiency and performance is the richer side of stoich. Since you want to go into the "weeds" per say I would suggest first grasping what the foundation of engine design really relates to which is "combustion"
Here's the advice I was given from the guy who new more than I'll likely ever to get the chance to figure out in regards to chryslers:
As for combustion:
"The most important stage is the preparation prior to ignition. When that is not right, as in this engine, the flame kernel doesn't grow fast enough and the result is a lower cylinder pressure at an equivalent crank position to a good kernel. The flame kernel is a growth of laminar conditions to around 20mm diameter, then the conditions change to turbulent flame and the speed of burn is much faster. What you have to know is its droplet size and the vaporization that influences the speed of both laminar and turbulent flames. So when an engine has large droplets its needs a lot of heat to vaporize them in the time available.
When I say to you guys that a particular engine hasn't got enough vaporization that is what I'm commenting on. How it looks inside the cylinder is like this, the AFR at ignition time is leaner than the average of the cylinder, lets say the average is 12.8 but at ignition time the AFR around the plug is AFR 14, the flame is going to grow slow and some of the flames energy is going to used in vaporizing the fuel thats not gas yet. So the flame kernel doesn't generate enough heat and you dont get a TAN plug, you also run the risk of extinguishing the flame if turbulence is poorly directed. The kernel uses up about 10% of the mixture then the conditions transition to turbulent. Once the flame is turbulent it cant be extinguished. However the conditions toward the remainder of the chamber ahead of the flame kernel front are still under vaporized, therefor requiring energy from the advancing flame front to vaporize mixture ahead of the front, not an efficient way to go. It's the burn angle duration between 10 and 90% that matters. Good burns are in the range of 15 to 25 degrees. Burns of engine with poor vaporization can be as long as 45 degrees. THAT'S WHY some engines make best power at timings like 38 to 45 degrees compared to another that best at 28 degrees."
Spend some time reading this:
This is the basic scenario of combustion in a cylinder from start to end.
1. Fuel is converted from liquid form to gas form. The amount of chemicals that are converted prior to ignition determines the regularity of the flame kernel and the strength of it. The temperature of the air during the compression time determines the number of fuel molecules that are heated via convection and turned to gas ready to burn.
2. Its not enough to just work on vaporization, the fuel molecules have to be spread throughout the chamber in a homogenous state. In designs for emissions it is deliberately done to create lean areas in the center of the chamber and a different mixture at the sparkplug and the outer areas of the chamber. The jury is out on the effectiveness of these approaches. For racing the approach is for an even spread of mixture because that allows a consistent tune throughout the chamber. In other words the design should prevent you from having to correct a lean detonation in one area by flooding it stupid elsewhere to bring the problem area under control.
3. The spark ignites the mixture present at the plug location. The initial flame growth is a laminar flame. Its laminar until its approx 12mm diameter. It takes approx 10 degrees of crank rotation at ANY RPM to achieve that diameter. The AFR of the mixture has hardly any effect upon the flame during its laminar stage. The burn speed is smooth and consistent during laminar flame. The burn speed is variable with the RPM but its not variable per event due to any factors like AFR Its varied a little bit by the vaporization because a droplet of un-vaporized fuel hit by the laminar flame front absorbs energy from the flame. Anytime energy is absorbed from a flame the flame speed is slowed. The arc energy is the most influential factor to the flame kernel. The energy field from an arc of the power of a crane HI-6 is approx 6mm radius. That’s the size of the laminar limit of the flame kernel. The power of the ignition is paramount to vaporizing the fuel load in that zone and achieving a stable flame kernel under high RPM conditions. The flame kernel is basically unstoppable once it reaches 12mm diameter.
4. At 12mm diameter and beyond the flame changes to a turbulent flame. Its starts to fold over on itself and looks like one of those bed donnas, quilts or cover things that are made of pockets of insulation type material. The pockets formed from stitching is what the flame starts to do, it forms that way from advancing and slowing rates within the original laminar form. Its like a stage of transitional flow in a pipe. It is at this time that mixture can be injected into the flame from areas like the squish bands. Injecting this fresh mixture into the fledgling turbulent flame supercharges its burn rate, its takes off ridiculously fast once you shove the squish in there.
5. The now fully turbulent flame is now at near TDC piston position and its burnt about 60 to 70 % of the mixture mass. Its at this point that you need to consider if your fuel has low octane chemicals in it(eg street unleaded) and you should start to gas them now so that if they decide to detonate they will at least be doing it on the power stroke and the detonation energy could be used for advantage. Does this wring a bell in your head about how one particular EMC winner for years was doing it?
6. By 14 to 18 degrees after TDC the mixture should be about 90 to 90% mass burnt and the cylinder should be at max pressure. It varies in position because of rod ratio and things like that that effect the mechanical efficiency of the engine and the location of your particular engines best peak pressure point. (PPP) But the PPP is in that Zone. That’s why we have computers to adjust the ignition timing so we can fiddle around in the top end and find the PPP.
7. Note at the PPP we have not completely burnt the fuel mass,we have only burnt 95% of it. That’s because we need the remaining 5% to chase the piston down the bore and to provide some energy to the exhaust pulse. If we don’t have enough energy for the exhaust pulse we cant scavenge the cylinder ready for the next charge. You can alter the exhaust pulse energy in many ways. Anything that alters the valve opening point, anything that changes burn time, anything that alters burn percentage at PPP, and the list would go on and on.
8. All this time some of the burn energy is going into the water jacket and some energy is transferring to the piston. If the piston is ascending, then that part of the energy is returned to the chamber and its used to keep improving the burn (hopefully, and not to start melting stuff) If the pistons descending then its converted by the crank/rod assembly to torque.
9. The unused energy from the burn (that which is not converted to torque etc) is exhausted as heat and pressure and speed of gas flow. We measure with thermocouples the temp and discuss in the pits if its rich or lean etc but in reality its rubbish. We should be discussing if we have enough exhaust retention or removal from the chamber and we should be discussing if the gas is CO or CO2 and we should be discussing if the gas temperature is assisting us in vaporization of the next charge and asking if there a better way that may have some advantages with less of the disadvantages of retention. Reading the spark plugs is a way to start discussing these factors.
That should keep you entertained for a while. Good Luck!
A good description. More to it than that. Rod to stroke ratio has proven to have a minor effect. For a street engine this is not a torque or power issue. Now a long rod tends to reduce piston slap.so the engine runs quieter.
The carburetor booster venturis begin the atomization and vaporization of the fuel. The atomization is a factor of the air speed and turbulence, which shatter the droplets into tiny droplets. To get a visualization, connect a timing light. Any cylinder will be fine. You just need the strobe light. With the air cleaner lid off and the engine running, shine the timing light down the carb venturi. You need to open the throttle a bit to get the main circuit supplying fuel.
Now if you can find a throttle body injected engine, do the same. The injection at about 15PSI instead of the carb at atmospheric, does a lot better job
of atomizing the fuel. A good part of why injection gives lower emissions and better fuel economy. Of the droplets that get into the cylinder, during the initial burn, only at the surface vaporizes. What burns is the vapor around the droplets. As combustion continues, the heat causes more of the droplet to vaporize. Large droplets probably do not burn and they finally vaporize in the hot combustion gases going out the exhaust valve.
So vaporizing the fuel and air extremely well before entering the cylinder should be a good thing right? Welll, not so fast. As we learned in school science class, a bit of alcohol on the back of your hand evaporated, vaporized, very quickly and felt cold. The vaporizing fuel takes heat out of the air to vaporize. This can and does cause the air to contract slightly. However, working counter to this is the vaporized fuel takes up more volume.
Now also in school.science we learned water is an amazing chemical. Its volume decreases as the temperature is reduced to freezing, but then it expands after becoming its solid state, ice. Further cooling causes it to expand. Now that water when heated expends in volume until it boils or vaporizes. This vaporization consumes huge amounts of enegery, heat. One liter of water at 205°F becomes 1,600 liters of steam at 213°F. I describe this as an indication of the volume change as the state changes, even with minimal temperature change.
The lesson here, we need some vaporized or gaseous fuel to start combustion with the rest in very finely atomized droplets. This is partly why first port injection and now direct injection work for economy and emissions.
Carburetors: atmospheric pressure.
Throttle body injection: about 15PSI
Port injection: about 50 PSI to 100PSI
Direct injection: 500PSI to 1500PSI
Then we get into swirl and tumble in the combustion chamber. 4 valve heads tend to produce tumble. Any effect this has on combustion falls away as the RPM increases. Swirl functions to benefit the combustion throughout the RPM range. 2 valve engines tend to produce swirl, but is affected by the port location in relation to the intake valve. In most cases judiscious grinding on the cylinder wall side of the bowl will create a port bias. This is the angle of the bowl compared to the valve stem. Swirl is mixture motion in the cylinder/combustion chamber. Quench also creates beneficial mixture motion. This mixture motion aids quick combustion requiring less ignition advance.
