Posted: July 27, 2011 in Data Aquisition

     Deciding what ‘needs to be done’, in other words, airflow changes or corrections, air/fuel ratio changes, fuel composition or octane including the use of alternative fuels, spark intensity, duration, and even advance or retard of the spark in relation to crank angle needs to be more than just a ‘theoretical’ choice.  One must not assume or guess on changes in any of these variables.  So, the question is asked,” How am I supposed to make logical decisions?”  Data acquisition is the answer.  Not only just the acquisition of data but the understanding and comprehension of data retrieved in order to make those logical decisions on ‘what needs to be done’ to achieve your goals.   

     Engineers use, as an example, pressure transducers to determine charge air pressure, combustion chamber efficiency from the rise in cylinder pressures in relation to the ignition of fuels at a specific crankshaft angle or position, and also the release of cylinder pressure into the exhaust as well as exhaust pressures in relation to the effects of scavenging.  Furthermore, the use of thermal sensors, referred to as pyrometers, are used to retrieve data on items such as charge air temperatures, thermal energy release from fuels as well as heat energy loss from exhaust gas temperatures to determine the efficiency of the engine assembly.  Without the retrieval of this data, changes made are nothing greater than a ‘guess’.  

     With increases in air volume or velocity by changing the intake manifold, changing cylinder head design, camshaft profiling, etc, it causes cylinder pressures to increase or decrease depending on the changes made.  An increase in spark timing (advance) leads to a more rapid rise in cylinder pressures, possibly too soon resulting in a pre-ignition event and also misfires which can be determined by utilizing the CA50 theory.  This theory describes the crank angle in which 50% of the fuel available in the cylinder is burned.  CA50 should occur ideally at 7.5deg ATDC of crankshaft rotation which would result in optimum cylinder pressures.  So, this theory helps explain why higher octane fuels, or even some alternative fuel blends, require more spark advance. 

     Combustion analysis refers to the process of evaluating thermodynamic performance of an engine which is the effect of utilizing chemical dynamics of different fuel compositions.  Crank-angle-resolved cylinder and manifold (intake and exhaust) pressure measurements are the primary inputs for combustion analysis.  So, why is combustion analysis so important?  Simply, torque is the twisting force that rotates the engine and is produced in the combustion chamber.  Fuel is introduced in the cylinder as chemical energy and, as it is ignited, changes state into thermodynamic energy.  This change of energy state causes cylinder pressure to rise, therefore creating enough force to move the piston in the downward direction.  If CA50 is created too soon or too late, maximum cylinder pressure may not be possible. 

     In-cylinder processes establish the maximum amount of available torque that can be extracted from an engine.  So, air to fuel ratios, and ‘when’ the spark occurs, dictates how much torque could possibly be produced.  Leaner engines, in order to produce more torque, require more energy input as fuel and furthermore require a longer burn time, or thermodynamic cycle, to release thermal energy from chemical.  Richer air to fuel ratios require either more spark advance, which results in a longer burn time to promote complete combustion, or an air to fuel ratio change to achieve better combustion efficiency. 

     All engines should be designed around their own specific combustion efficiency strategy, or thermodynamic cycle.  Combustion analysis enables a more detailed dissection of what is occurring in the cylinder.  Also, another form of torque loss is friction of the rotating assembly therefore combustion analysis allows for ‘friction quantification’ of an engine.  Improvements in MBT improve torque output.

We are fortunate to have some wonderful writers contributing to our publication.

Here is one of them by Stephen E. Mole


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