Combustion Characteristics Considered

Throughout this web site you will see references to “Combustion Efficiency” (not to be confused with Volumetric Efficiency, both of which improve performance).  What is it?  In short, it is the engine’s ability to convert the chemical energy in the fuel to kinetic energy at the flywheel.  Science has created charts to convert one form of energy to the equivalent of another form.  For example, gasoline is rated in BTUs per volume.  Engines are rated in Horsepower.  The conversion chart suggests it takes 2544.4 BTUs to produce 1 horsepower of work.  An engine will produce less horsepower than the BTUs in the fuel it consumes due to Faraday’s First Laws of Thermodynamics — there are always inherent losses.  This becomes a mathematical formula where the efficiency result = Combustion Efficiency:  How much fuel does it take to make 1 horsepower in an engine?  On a dyno sheet, it is the BSFC value — Brake Specific Fuel Consumption — where lower numbers = better efficiency.  Here’s a higher-level scientific look at the process:

Engine power is the result of this chemical-to-kinetic conversion.  The 1970 Boss 302 Mustang was able to produce 290 horsepower from a 5.0 liter (302 CID) engine, and delivered ~10-ish MPG in combined driving.  The 2021 Mustang GT produces 460 horsepower from its 5.0 liter engine, yet delivers 25 MPG.  Same size engine in a similar size vehicle, but radically different efficiencies.  Both power and economy (and emissions) have been dramatically improved on the new Mustang.  In other words, the combustion efficiency is much better in the 2021 version.

The total vehicle is capable of burning 98% to 99% of the fuel.  This means virtually no toxic HC or CO emissions coming out the tail pipe.  This reflects combined efforts between the engine and catalytic converter working in partnership.  However, the US EPA claims the engine is only converting between 18% and 25% of the BTU energy in the fuel to kinetic energy at the crankshaft.  If we look at conversions in other sciences, an old-fashioned brushed DC electric motor is around <80% efficient at converting the electrical energy to kinetic energy.  A modern brushless DC (BLDC) motor, on the other hand, can convert >95% of the electrical energy to useful work.  Why are internal combustion engines so incredibly inefficient?!?  (Which brings up another question, why has the world accepted such dismal efficiencies for over a century??)

Chemistry

The US EPA claims new gasoline engines, using gasoline for fuel, are typically around 20% efficient.  The same gasoline engine burning propane is around 44% efficient.  If using natural gas as the fuel, that same engine becomes around 60% efficient!  It looks like gasoline is a lousy fuel for the gasoline engine!!

Think about that:  an engine can go from 20% to 60% efficiency just by changing the fuel.  The reason is two-fold:

1. Gasoline is a liquid fuel, and liquid fuel doesn’t burn.  Both propane and natural gas are gaseous/vaporous fuels.  The vaporous fuels can begin combustion at the point of ignition without needing an endothermic vaporizing step (which gasoline requires).
2. Propane is a 3-Carbon hydrocarbon molecule — C₃H₈.  Natural gas is a 1 Carbon molecule — CH₄.  Gasoline molecules range from Pentane — C₅H₁₂ — to Dodecane — C₁₂H₂₆.  Smaller molecules can vaporize and completely oxidize/burn in less time than larger ones.  It’s like burning saw dust versus a log.

Fractured HC molecule.

Fracturing an oxygen molecule.

Oxygen fractures, then combines with a single carbon to form CO, then with an additional oxygen atom forms CO₂.

Hexane (C₆H₁₄, one of the light compounds in gasoline) vaporizes at 135 degrees F and takes less than 1 millisecond to fully oxidize once ignited.  Dodecane (C₁₂H₂₆) vaporizes at 435 degrees F and takes over 33 milliseconds to burn.  When the spark plug fires, most of the light elements like Pentane and Hexane will already be vaporized.  They are what the spark plug ignites.  As these light compounds burn, they release heat.  Some of this heat is used to vaporize the heavier fuels.  Some of it is used to ignite other vaporized fuels (ignition is endothermic).  If there’s any left over, it contributes to power.  As more of the total fuel vaporizes, it catches fire, releasing heat.  Eventually, the released thermal energy contributes little to vaporizing and igniting, and mostly to making power.  The majority of usable energy is released late in the combustion cycle.

Molecular representation of common straight-chain hydrocarbon (HC) fuel molecules.

However, there is a catch.  There is only a small window of opportunity to actually harness this energy before the exhaust valve opens and begins evacuating the cylinder.  Exhaust manifolds are extremely hot because the gasses passing through them are still burning.  This means the fuel is still releasing heat energy after it’s too late.

Math

At highway speeds the engine is spinning at around 2400 RPM, or Revolutions Per Minute.  This is 40 revolutions per second (2400/60).  Therefore, it takes 0.025 seconds (1/40 = 25 milliseconds) for the engine to complete 1 full revolution.  The power stroke takes half of a stroke, or 12.5 milliseconds (25/2).  The ECU will fire the spark plug before the piston reaches Top Dead Center (TDC) on the compression stroke, and the exhaust valve opens before the piston reaches Bottom Dead Center (BDC) on the power stroke.  Optimistically, the engine has no more than 12 milliseconds to harness energy between the spark plug firing and the exhaust valve opening.

Dodecane requires 33 milliseconds to burn once it has been vaporized and ignited!!  It requires 435 degrees to vaporize at atmospheric pressure, but when compressed (remember the compression stroke is a ~9:1 ratio) it takes higher temps (think “pressure cooker”).  Even if a dodecane molecule were fully vaporized when the spark plug fired (highly unlikely), and happened to be smack in the middle of the spark plug electrodes, it would still be burning when the exhaust valve opens 12 ms later.

If you ran a line of gasoline on the floor and lit it at one end, using a high speed camera you would find the flame moves along at 41.5 cm³/sec.  Considering the size of the average engine’s cylinder bore, it takes ~1 ms for the flame front to reach the outer edges of the combustion chamber.  So about 1 ms into our 12 ms window we at least have the light vapors burning.

Vaporizing the fuel is a function of heat over time; more heat means less time required.  Therefore, there is not much vaporizing going on early in the combustion cycle.  As more heat is released, more vaporizing occurs, and at an accelerated rate.  The black soot inside exhaust passages is the result of fuel that never did vaporize in the combustion process, exiting the exhaust valve as a liquid droplet; evidence that not all fuel fully vaporizes (let alone burns) within the Magic Window.

You can have an open container of gasoline exposed to oxygen in the air.  It does not instantaneously catch fire.  It requires a spark, or lit match to trigger the burning process.  The energy from the spark or match contributes to an Endothermic Reaction.  In other words, it takes energy to split the oxygen atoms, and to peel hydrogen atoms away from the stable HC fuel molecule; then eventually split the HC carbon chains (see above illustrations).  Once the atoms are freed, they readily re-combine (oxygen with hydrogen or carbon), releasing heat — an exothermic reaction.  HC + O₂ -> H₂O + CO₂ is the conversion formula.  At the start of combustion, the oxygen in the air, and the hydrogen and carbon in the fuel are stable.  They combine only when destabilized — the endothermic part.  This is why much of the heat released from burning the fuel early in the combustion cycle is used to start more of the fuel on fire (and does not contribute directly to making power).

Physics

If you try to loosen a rusted bolt with a wrench, with the wrench pointing away from you at the 12:00 position, pulling it towards you cannot possibly budge the bolt in either direction.  If you move the wrench to the 9:00 position, you get the maximum leverage on the wrench to loosen the bolt.  A piston at TDC cannot contribute to engine rotation.  Downward force only puts pressure on the connecting rod and bearings.  The piston has maximum leverage at 90 degrees After TDC (ATDC).  Sort of…

As the piston moves away from TDC, it gains more leverage on the crank, up to the 90 degree point.  However, as it does, the volume inside the cylinder increases.  All else being equal, as volume increases, pressure decreases.  Since the fuel is still burning, creating heat and thus pressure, the diminishing pressure effect is somewhat offset.

There comes a point when the piston is moving so fast that it actually outruns the pressure front.  An engine with a 100 mm stroke will have a piston speed of (100 mm / 12.5 ms =) 8000 mm² per second, or 800 cm²/sec — that’s already >19X faster than the flame propagation speed of gasoline (800/41.5).  But wait, it gets much worse!  The rate of volume increase will be (800 cm²/sec) X cylinder area (Pi X Radius²).  A 90 mm bore will be increasing the volume (at 90 degrees ATDC) at a rate of 3.1415 X (90/2) X 800 = 5,089,376 cm³/sec!  This is over 122 thousand times faster than the flame propagation rate of burning gasoline (5,089,376/ 41.5)!!  Engineers deem peak cylinder pressure at 17 to 18 degrees ATDC as ideal (officially named Critical Crank Angle, or CCA).  This harnesses the pressure at the most advantageous angle of rotation, while accounting for the increase in volume as the Power Stroke progresses and the piston speeds up.

With that in mind, we really don’t have 12 ms to harness energy, we have at best 45 degrees of engine rotation; or about 3 ms!  Approximately 80% of the power delivered to the crankshaft comes from the first 45 degrees of crank rotation in the power stroke.

Conclusion

Upwards of 80% of the energy conversion from chemical to kinetic occurs within the first 45 degrees of piston travel in the power stroke, yet only ~15% of the chemical energy in the fuel has contributed to power within that time frame.  A smidgen of the energy is released from the fuel within the magic window when it can harness the most power.  The ONLY way to improve combustion efficiency is to release more of the chemical energy within this magic window.  This means getting the fuel to a combustible state sooner.  This means igniting more of it sooner in the combustion process so more of the chemical energy can be converted to heat within our minuscule magic window.  It means reducing endothermic losses.

To improve Combustion Efficiency, you must be able to convert more of the chemical energy in the fuel to thermal energy, then kinetic energy within the magic window:

• Improve the vapor to liquid ratio prior to ignition
• Improve the homogenization between the fuel and oxygen in the air
• Increase in-cylinder turbulence to help carry the flame front to the fuel, and fuel to the flame front (swirl)
• Reduce endothermic requirements to initiate oxidation
• Deliver only the amount of fuel required to satisfy the combustion event (no overly-rich mixtures)
• Ignite the charge so peak cylinder pressures occur at 17-18 degrees ATDC (spark timing)

Here are some points to ponder:

• Heating the fuel promotes a higher vaporization rate earlier in the combustion cycle.  Less energy is spent in the vaporization process, leaving more to power the vehicle.  Pre-vaporized fuel catches fire earlier in the cycle, delivering more thermal energy to act on the expansion medium within the magic window.
• Magnetics de-cluster the fuel to promote faster and easier vaporization.  Similar effect as heating the fuel.
• A more intense spark can ignite more of the fuel earlier in the combustion cycle.  PDI broadcasts the spark energy outwardly into the combustion chamber.  Far more fuel is ignited by the PDI spark than just what’s between the electrodes.  This accelerates the flame propagation rate significantly.
• Combustion accelerants — like ozone, HHO, or perhaps certain fuel additives — speed up the flame propagation rate of the primary fuel.  The faster burning accelerant carries the flame front to the far reaches of the combustion chamber.  This accelerates the flame propagation rate, and actually generates eddy currents in the charge as it blasts through.  This turbulence helps to better mix the air & fuel.
• Better homogenization more thoroughly intermixes the fuel with the oxygen, making sure that when a fuel molecule does vaporize, and is exposed to the flame front, it can ignite.  It also helps smooth hot pockets, transferring thermal energy to the expansion medium more efficiently.  Swirl port induction (in some cases to include “Tornado” type devices, throttle body spacers, or the Gadgetman Groove) induces more mechanical blending and homogenization of the air and fuel.
• Intake and exhaust system designs can induce greater swirl action at part throttle, and better homogenization of the air and fuel.
• Electro-chemically destabilizing the air and/or fuel before the ignition event will reduce endothermic losses in the ignition/oxidation process.  Ozone (O₃) is a far less stable form of oxygen than ambient air oxygen(O₂).  It virtually falls apart in the combustion process; less endothermic losses, and faster response time (ready to combine with hydrogen and carbon atoms sooner).
• Higher compression ratios push fuel and air molecules closer together when the spark plug fires.  This means there is less distance for the flame to travel before running into another fuel and oxygen molecule.  In a sense, this speeds up the burn.
• Higher compression ratios = higher expansion ratios.  This extends our “magic window”, giving more time to harness the energy from the burning fuel.
• Ions pre-combustion = good; ions post-combustion = bad.  Grounding the exhaust system electrically neutralizes (shorts out) the ionic charges in the exhaust pipes allowing the gasses to move more freely.  Many of the 100 MPG carburetor designs interfaced the intake charge with the exhaust gasses.  Wasted energy from the exhaust gets recycled back into the intake charge.  Most of that energy is thermal, however, some of it is electrical.  Modern turbochargers utilize a similar recycling of energy concept.
• Crankcase gasses typically contain stuff that doesn’t burn (water vapor, carbon dioxide, etc), and heavy oily HCs that burn even slower (with much higher vapor points) than the heaviest compounds found in the fuel.  In other words, the PCV system puts elements into the combustion process that deter efficient oxidation even more than the worst chemicals in the fuel.  PCV catch cans and the Smart Emissions Reducer reduce that negative effect on combustion.

FE-NR

MPGenie Basics 051 Training - Combustion Characteristics Considered Part 1

MPGenie Basics 051 Training - Combustion Characteristics Considered Part 2

Return to MPGenie