American writer Mark Twain once said "It ain't what you don't know that gets you in trouble.
It's what you know for sure that just ain't so." This is especially so in the case of what even well-educated engineers know about how IC engines work. Often what they know is based on an “idealized” p-V diagrams, like the one shown here from NASA. While useful as a teaching aid on the function of the IC engine they are a
“fundamentally incorrect oversimplification”. However, a critical examination of this theoretical ideal can give us at least three clues as to how we can vastly improve the IC engine concept.
1. This Ideal depiction of how an IC engine works shows the combustion process as a
“constant volume process”. But this would require the piston to instantly stop at TDC so the volume would remain constant while the flame front moves through the chamber. However, this would cause the piston to collide with Newton's First law of inertia and would cause lots of things to break.
It just ain’t so that combustion is a constant volume process. However, Professor Oppenheim explained that increasing the dwell time after ignition giving combustion time to burn through the A/F charge would significantly increase thermal efficiency. He also suggested using homogenous charge compression ignition would also improve thermal efficiency.
2. Most p-V diagrams show the power stroke as an “adiabatic process”.(No heat loss) That would be ideal because the concept of an IC engine is to turn the expansion of the hot A/F after combustion into work. But, that too just ain’t so. If the heat of combustion did not make the engine hot it would collide with the second law of thermodynamics, which in its most understandable wording states “Heat always flows spontaneously from hotter to colder regions of matter.” If none of the heat of combustion was lost during the power stroke there would be no need for a cooling system and more of the heat’s energy could be converted into work. Professor Oppenheim correctly points out that heat loss to a cooling system could be vastly reduced with more suitable materials than aluminum which needs to be cooled to just over 200 C, or it loses its strength. The combustion chambers of all IC engines have to contain a flame front which is over 2.000 C, but aluminum melts at just 660 C, big ouchie! Thankfully
aluminum is good at moving heat into the cooling medium because it has a thermal conductivity of
237 W/m K. We found a better material,
304 stainless steel, which has a working temperature of over
700 C and a thermal conductivity of just
16.2 W/m-K, which is 15 times more insulative than aluminum. It's like putting on that insulated jacket to slow your heat loss.
Another advantage of 304 SS is carbon will not condense on it and it is very reflective of radian heat. Another nice advantage of 304 is the combustion temperature if HCCI is used is only about 600 C, not a problem for 304 liners. This lower temperature of combustion also eliminates the danger of igniting the nitrogen into NOx.
3. Many p-V diagrams also depict heat rejection with a line that is 50% the length of the combustion line, suggesting 50% efficiency. This too just ain’t so. Most Otto cycle engines reject 75% or more of the heat energy of combustion through friction, cooling, and exhaust. However, Professor Oppenheim argued that if the flame front method of combustion were to be replaced by
Homogenous Charge Compression Ignition, or HCCI. He also consulted with the design team regarding other improvements so that the IC concept’s efficiency could be vastly improved. Electronic control of HCCI is an area where we need technical help.
In his books “Dynamics of Combustion Systems,” and “Combustion in Piston Engines”. Professor Oppenheim outlined the importance of electronic controls to manage combustion if HCCI is to be successful. He also predicted that HCCI would never work outside the lab in any 4-stroke engine. So far 20 years later that is still true. The ideal environment for a well-managed HCCI would be in a 2-stroke with chamber expansion dynamics which closely match the dynamics of combustion and surface temperatures hot enough to prevent the quenching of the combustion event.
A more ideal Dynamic.The last image is a graph of the Rad Cam’s piston or expansion dynamic as compared to that of a crank-driven engine. The piston does not stop at TDC its dynamic is closer to the ideal of allowing combustion to be a “constant volume process” than a crank-driven piston. This is illustrated by the piston dynamic graph. The red and pink acceleration line show the crank pulling the piston away from TDC at over 200% of the acceleration rate of the cam. This acceleration rate diminishes to the cam’s rate at 46 degrees. At just 75 degrees, its acceleration reverses when the piston reaches its terminal velocity and starts to slow. In contrast, the Rad Cam piston reaches its terminal velocity at 90 degrees or mid-stroke and then starts to slow at the same acceleration rate.
The blue position lines on the same graph further illustrate this point. For example, when the crank rotates just 45 degrees the piston has descended 30% of the stroke. This means that the top 45-degree sweep post-TDC of the volume has increased by 30%. This is not even close to the ideal.
However, the same relative rotation Rad Cam’s piston moves it just 7.5% of its stroke. This means that the top 45-degree sweep post TDC of the volume has increased by only 7.5%. This is 400% closer to the ideal of making combustion a “constant volume process”.