Why do gaseous-fuel generators (Propane, NG) have such a short lifespan compared to liquid fuel (diesel) generators?

In diesel engine fuel is injected in the form of fine atomized liquid particles at the end of the compression stroke. The phases of combustion of diesel can be seen here. http://www.dieselnet.com/tech/diesel_comb.php In a gas powered engine it isn't purely gas cycle bus it is a mixture of diesel and gas combustion. As gas cannot auto ignite at the temperatures of compression some additional source of ignition is required. A small fraction of diesel injected at the end of compression process serves in igniting the gaseous mixture. Additional details can be seen in the following link http://en.wikipedia.org/wiki/Bi-fuel_vehicle#Diesel_conversions Because of the decreased ignition delay in case of gas cycles, rate of power release is high. This can be studied with the help of graphs. The detailed analysis of heat release rate and cylinder pressure rise helps us in getting through to the correct answer. Here is the heat release chart for a diesel engine. Here is the graph of an DDF (diesel dual fuel) engine that runs on a mixture of diesel and natural gas. Here is the graph for heat release in this case. The graph gives the increasing fraction of gas in the fuel fired as 'Z' . you can observe that as the proportion of gas is increasing, the heat release rate is increasing tremendously. For a diesel cycle, the peak heat release observed is 14.5 J/0.25 degree crank rotation. In the case of gas cycle it is seen that the heat release rate is 72 J/degree i.e, a average heat release of 18 J/ 0.25 degree. From this observation we can observe that a very large amount heat is released in the case of a gas engine against large amount of heat release in case of diesel engine per degree crank rotation. Heat release rates are almost proportional to engine cylinder pressure rise. Thus instantaneous pressure in gas engine is high when compared to diesel engine of same capacity. This implies that the rate of change of force acting on a piston of gas engine is much higher. Impact load acting on an engine is much higher in case of gas engine. This leads to higher instantaneous stresses in parts of gas powered engine. Due to the cyclic impact load acting which induces sharp rise in stresses over a very short span of time, engine parts in gas engine fail much earlier than of those in diesel engine.