Before we can understand the nuances of what Pulse-Jet does we need to consider the differences between compression and spark ignition. Spark ignition “does what it says on the tin” the heat from the spark heats the molecules of fuel adjacent to the spark to a point where they will react with the oxygen, this reaction spreads through the combustion chamber as a single flame front, each molecule heating the adjacent one as it reacts, this continues until it runs out of fuel, oxygen or the front reaches molecules it can’t put enough heat into, these are normally the boundary layer that sit against the fixed surfaces, cylinder head, cylinder wall and piston. Compression ignition is also fairly self explanatory, air is compressed in the combustion chamber usually around twice the compression as in a spark ignition, high pressure fuel is then injected into the combustion chamber, the combined energy of the compression and high pressure injection gives each fuel molecule enough energy to react.
Spark ignition engines need to run with the correct amount of fuel to oxygen for a continuous flame front. This is known as a stoichiometric mixture, compression ignition engines don’t need to be stoichiometric as each molecule has enough energy to react and isn’t reliant on some of its neighbours energy.
What does all this mean?
Spark ignition engines need to have both fuel and air regulated to maintain a stoichiometric mixture, the act of regulating the air reduces the pumping capacity and therefore efficiency of the engine, compression ignition engines only need to regulate the fuel as they will run on very lean mixtures, but there is a problem, the available oxygen is very reactive and with the temperatures and pressures experienced it reacts with the nitrogen to produce toxic oxides. In summary and a little glibly, spark ignition engines heat up the planet by means of co2 and compression ignition engines poison the planet by means of nitrogen oxides.
Compression ignition engine developers have come up with a number of ways to help reduce this, the main one is exhaust gas recirculation, EGR, by feeding the engine a percentage of oxygen depleted exhaust gas instead of air as an expandable medium, the available oxygen is reduced and so are nitrogen oxides, EGR also helps reduce the spikes in burn temperature all of this without throttling the intake and reducing pumping capacity, some compression ignition engines recirculate up to 50% of their exhaust gas. Spark ignition engines could really benefit from EGR which would reduce the throttling necessary and improve pumping efficiency however with percentages more than 5% the single flame front tends to fail.
Ok! at last to the point
PJI has a natural inbuilt EGR, the internal combustion chamber is exhaust gas locked at the end of the exhaust stroke and the percentage of EGR is proportionate to compression ratio, this natural EGR helps stabilise the burn temperature and reduce the nitrogen oxides, the pressure builds vey quickly in a confined space, the emerging explosion from the PJI ports ignites the main combustion chamber with turbulence that breaks up the boundary layer and by definition has multiple flame fronts, this multiple flame allows for much more EGR to the main chamber improving the pumping efficiency of the engine and reducing co2 emissions, these advantages make PJI the most efficient method of igniting renewable bio fuels which will form part of the future in bridging the gap to electric if indeed the gap gets bridged!