RATE OF HEAT RELEASE

In Piston Engines

The key to maximum performance of internal combustion piston engines is to match the speed of the motor mechanism to lightning fast combustion.   R. Diesel recognized this when he wrote:   "...The combustible is added in such a way, that no increase in temperature of the gases, consequent upon the process of combustion, takes place, .... After ignition, combustion should not be left to itself, but be regulated by an external arrangement, maintaining the right proportion between the pressures, volumes, and temperatures."

It has been a formidable technological challenge to achieve what the industry calls a "rate of heat release" that matches Diesel's prescription, particularly for the higher speed engines found in vehicles.   Yet to be accomplished, the difficulty of this task has stimulated a field rich in creative innovation.   Better control over diesel fuel injection can narrow the gap between the theoretical maximum and actual fuel economy.

Terminology
Certain chemically reactive mixtures can either burn or explode.   For example, a solid-fuel rocket can gradually release its energy or, if its nozzle plugs, it turns into a bomb.   Despite the common vernacular for how internal combustion engines operate, the terms "explosion" and "detonation" are not used here because they describe a destructive effect, an exothermic chemical reaction accompanied by a shock wave.   It is more accurate to use the words burn, combust, or any other word describing an exothermic chemical reaction without the destructive shock wave.   Detonation, when it exists, indicates an operational problem.

Time and Temperature
These concepts help define the rate of heat release in a piston engine.

1. As used in this website:
   
   
   • Color indicates the approximate temperature per the scale on the comparison diagram.
     
     
   • High temperature is the maximum temperature of the air due to compression.
     
     
   • High temperature is distinct from the yet hotter flame temperature of the chemical reaction between fuel and oxidizer.
     
     
   • Autoignition temperature is the minimum air temperature at which a particular fuel will ignite, without an external ignition source such as a spark or flame.
   
   
2. Although each individual chemical reaction occurs at the hot flame temperature, it yields only a small quantity of heat.   The sum total of these reactions is the total amount of heat released.
   
   
3. The time that it takes for each individual chemical reaction to occur is so short compared to the movements of the engine mechanism that the mechanism can be considered to be relatively motionless while each reaction occurs.   In effect, the appearance at the flame temperature of each unit quantity of heat due to each individual chemical reaction is practically instantaneous compared to the engine mechanism.
   
   
4. The fuel injection equipment controls the spread over time of the total number of individual chemical reactions, resulting in the rate of heat release.
   
   
5. Heat distribution throughout the working fluid depends on how well the reaction products are mixed.

Comparison of Combustion at Constant Volume, Pressure, and Temperature
The consequences of different rates of heat release can be predicted, compared, and contrasted, starting from the perfect gas law.   This law is pV = mRT, where p is pressure, V is volume, m is mass, R is the universal gas constant, and T is temperature.   Since m and R will stay nearly constant during combustion, predictions with p, V, and T can be made.   For example, if combustion occurs while volume does not change, termed "combustion at constant volume" or CV, then to maintain the equality p and T must vary in ways that can be predicted.   The same is true for combustion at CP (V, T) and CT (p, V).

In other words, the perfect gas law can be used to predict engine thermal efficiencies for different rates of heat release.   Limitations on the technological ability to control the rate of heat release translate into limitations on fuel economy and emissions.   The rate of heat release can be more precisely controlled by a programmable diesel fuel injector.

The diagram shows a colored temperature scale on the left.   The differences between combustion at constant volume, pressure, and temperature are graphically visualized.   As stated below for the gasoline engine, the CV cycle scale is inaccurate but is still illustrated this way to highlight its differences from the CP and CT cycles.   Color is used in Carnot's thermal efficiency equation below (shown also on the Ideal Heat Engine page) as an aid to clarifying why the CT cycle is superior to the CV and CP cycles.

Fuel Preparation
For this discussion, the fuel is a hydrocarbon and the oxidizer is the oxygen in ordinary air.   Conveniently stored in a low pressure tank as a liquid, the fuel must undergo physical preparation before it will burn.   Since, in general, liquids do not burn, the function of the engine fuel sub-system is to a) at the right time vaporize the right amount of liquid fuel, and b) mix the vapor with oxygen-containing air in the chemically correct proportion.   Once step b is completed, then the mixture is ready for ignition.   Both vaporizing and mixing each take time and must occur in sequence; the importance of their influence on the rate of heat release after ignition cannot be overemphasized.   How they are accomplished is one of the ways piston engines are separated into their two main classes.

Engine Classes
There are two main classes of piston engines; the familiar spark-ignition gasoline engine and compression-ignition diesel engine.   In the gasoline engine, fuel is vaporized and mixed with air before (relatively) low compression occurs.   In the diesel engine, fuel is vaporized and mixed with air after (relatively) high compression occurs.

Gasoline Engine or Spark Ignition Engine

• Because the gasoline engine gives its liquid fuel enough time to vaporize and mix well with air, this engine holds the advantage in mixture preparation uniformity, critical to supporting complete combustion.
  
  
• If the mixture reaches its autoignition temperature prematurely, it will detonate destructively.   This is prevented by limiting the engine compression ratio.   Because this relatively low compression limits the cycle high temperature to less than the autoignition temperature, it limits the maximum theoretical cycle efficiency possible from a gasoline engine.
  
  
• Fuel octane rating is a measure of the fuel's higher (than cetane) autoignition temperature.   Octane helps resist detonation / pinging / spark knock (which if left untreated can eventually destroy an engine).   Designing an engine for higher octane fuel permits higher compression ratio.
  
  
• The mixture of air and octane fuel vapor should not burn until the flame front reaches it.
  
  
• Due to the speed of a "flame front" as it proceeds away from the spark plug, combustion closely approximates combustion at constant volume (CV), the leftmost cycle on the comparison diagram above.
  
  
  • (For a real gasoline engine which must avoid detonation, the comparison diagram above is relatively inaccurate for the CV cycle because it implies that compression proceeds to the same high level as for the CP and CT cycles.   In fact, the cycle high temperature must always be less than the autoignition temperature defined by the fuel octane rating.)
    
    
  • The rate of heat release is so fast that the energy total raises the temperature T of the compressed working fluid from high to very high, which causes pressure p to rise in general accordance with the perfect gas law.
    
    
  • Engine work output equals force multiplied by distance traveled.   While force on the piston rises rapidly with working fluid pressure p, CV combustion means that the distance traveled by the piston is zero and therefore work output is zero.
  
  
• Thermal efficiency is relatively poor due to:
  
  
  • Low compression ratio which limits the cycle high temperature and
    
    
  • Escape of very high temperature heat away from the working fluid and out the radiator and exhaust.   Re-stated in Carnot's archaic but direct language, this major heat leak is a "useless re-establishment of equilibrium in the caloric."

Diesel Engine or Compression Ignition Engine

• Because the diesel engine injects its liquid fuel directly into the combustion chamber, it offers control over the rate of heat release, critical to achieving maximum fuel economy.
  
  
• To promote the compression ignition by which this engine operates, its high compression ratio intentionally raises the temperature of the gas working fluid.
  
  
• Fuel cetane number is a measure of the fuel's lower (than octane) autoignition temperature.   Cetane promotes autoignition or compression ignition, necessary for the engine to operate.
  
  
• The mixture of air and cetane fuel vapor should burn as soon as it forms.
  
  
• Combustion is approximated as a mixture of constant volume followed by constant pressure, referred to as a "limited pressure" cycle.   Specific engine and fuel injector characteristics determine the mix between the leftmost and middle cycles on the comparison diagram above.
  
  
  • The rate of heat release is less than that of the gasoline engine but it is still too fast, raising both temperature and pressure, incurring significant heat losses at high temperature by uselessly re-establishing equilibrium in the caloric.
    
    
  • Work is done during the constant pressure portion of the combustion because the combustion is slow enough such that the distance traveled by the piston is greater than zero.
  
  
• The critical defect in existing fuel injection equipment is its generally ON-OFF telegraphic nature:   either liquid fuel is allowed to squirt from the nozzles at maximum velocity or not.
  
  
  • It takes time for the liquid to vaporize and mix before it will autoignite, a period referred to as ignition delay.   The sum total of mixture that initially develops then burns very quickly.   Just like in the gasoline engine, this can be approximated as CV combustion.
    
    
  • For the remainder of a single event of sufficient duration, the remainder of the liquid is injected into an existing fireball.   After vaporizing and mixing, this portion of the heat release can be approximated as CP combustion.
    
    
  • The heat leak during both CV and CP combustion contribute to a useless re-establishment of equilibrium in the caloric.

Best Fuel Economy: Constant Temperature Rate of Heat Release
When R. Diesel conceived his engine over a hundred years ago, heat engines were large and their rotational speed was relatively slow.   Remarkably and despite how crude his equipment had to have been, his engine of 1897 injected refined liquid petroleum and was able to attain 26 to 30 percent thermal efficiency, wildly successful when compared to the steam engines and "explosion engines" of the day.   Re-stated, the steady rotational speed was still so slow (compared to the physical and chemical processes of combustion) that mechanical fuel injection equipment could be constructed such that the rate of heat release approached CT combustion.   Significantly, he commented in his test report that combustion was very quiet, as it still should be.   His test engine did not have the benefit of turbocharging or charge air cooling but was only the power cylinder.

To put Diesel's achievement in perspective and illustrate the difference in development potential, current gasoline engines attain a 25 to 30 percent thermal efficiency.   After more than a century of intense engineering development effort, it is reasonable to conclude that the efficiency potential of gasoline engines is at its maximum since it cannot exceed Diesel's crude prototype.

The bottom line:   Using magnetostrictive electro-technology, the Great Plains Diesel Technologies, L.C. programmable diesel fuel injector can partially or fully open very quickly and then just as quickly be partially or fully closed.   Generally, such a rate shape should help reduce the heat leaks during CV and CP combustion, making them more closely approach CT combustion in a high speed truck or bus engine.   The best programmable rate shape depends on engine specifics.   The Great Plains injector has the control feature that will improve engine fuel economy while reducing emissions.   From inside a small package, it can do this while surviving on an engine cylinder head and can be retrofitted as an upgrade to the existing fleet.