I mentioned n-heptane in my earlier post as being the only light alkane having an auto ignition temperature lower than n-octane. For those who don't know, n-octane is the straight chain isomer of C8H18 and n-heptane is the straight chain isomer of C7H16. Being so similar in structure and molecular weight, n-heptane and n-octane have very similar, although not identical, chemical and physical properties. This means we should expect both to behave in similar ways under a given set of conditions.
It is interesting to note that n-heptane is commonly used in experiments as a surrogate fuel to diesel because it has the same cetane number as diesel (at least European diesel)
Reference 1. The use of a single species of molecule as a surrogate for the broad spectrum of molecular species found in diesel fuel makes the study of certain properties of compression ignition engines easier to perform and analyze. The combustion properties of n-heptane are not identical to diesel (e.g., n-heptane has a lower auto ignition temperature) but they are close enough to make it a viable surrogate.
So why have similar experiments not been conducted with n-octane as a surrogate? Primarily because the chemical kinetic properties of n-heptane have been more widely studied than most other hydrocarbons
Reference 2 so there is a greater quantitative knowledge of its behavior. It is reasonable to conclude based on structural similarities that n-octane would show results comparable to n-heptane if used as a surrogate.
What can we conclude from this? Increasing the n-heptane or n-octane content of diesel fuel has the potential, owing to the lower auto ignition temperatures, to improve cold starting for reasons discussed in my earlier post and yet without adversely affecting engine performance, owing to comparable cetane numbers.
Does this mean that everybody should go and add gasoline to their diesel? No it does not. Gasoline is a complex mixture of hydrocarbons that has bulk chemical and physical properties divergent enough from those of diesel to seriously adversely affect engine operation. What it does mean is that, in principle at least, there is validity to the claims of the manufacturer of the fuel additive that its product improves octane levels which in turn improves cold starting. Clearly the produce would have to be tested under controlled conditions to see if it in fact lives up to its claims, but the principles
behind the claim are solid.
wned: the Duramax long enough to realize it was a piece of garbage." :rules:
