Attention: TDR Forum Junkies 
rbattelle
TDR MEMBER
I'm a huge fan of technical threads, and I'm sitting here with a day off and got the itch to start one. So I thought I'd point out some interesting results from NACA Technical Memorandum No. 797 (available free here ).
Of particular interest is the autoignition temperature of the fuel. Wentzel points out that heating the cylinder air charge (end gas) to temperatures above about 350*F is "of no interest", since by that point the fuel droplets have almost completely vaporized. Significantly, the autoignition temperature of the resulting charge is much less than 300*F. What's my point? The point is, at the point of maximum compression the autoignition temperature of the mixture is remarkably low.
Also of interest are the results of Wentzels equations for rate of droplet vaporization. They indicate that at TDC, fuel droplets are fully vaporized within 0. 00106 second, even if ignition does not take place. If ignition does take place, the droplet temperature reaches 300*F within 0. 00055 second (recall that we need far less than 300*F for autoignition to occur). In essence, Wentzel is trying to quantify ignition lag. Of course, actual ignition lag is quite a bit longer, since there are some other factors that also come into play; ignition lag is typically on the order 0. 007 seconds. Why the discrepancy? Some of it can be explained by "chemical lag", which is just a natural finite time for the chemical reaction to occur. It is interesting to note that bomb experiments indicate little or no relationship between ignition lag and injection pressure.
Where does the remainder of the lag come from? It seems a lot of it comes from the fact that new fuel being introduced has a cooling effect at the spray core, which reduces the fuel temperature below the optimal vaporization and autoignition temperatures. Much of the lag we see in the Diesel, then, can be attributed to the heat transfer rate between the end-gas and the fuel spray.
So where does this bring us? Well, Wentzel found that detonation is most likely to occur when the injection period exceeds the ignition lag. Translation: injection time must be kept as short as possible. How do we decrease injection time? Increasing fuel pressure won't work, because our injector is an orifice. In an orifice, mass flow rate is independent of upstream pressure until upstream pressure falls below about 1/2 of ambient pressure (pressure in the cylinder during injection). So increasing fuel pressure will not result in any additional fuel entering the cylinders.
Then the only way to do it is to increase the orifice diameter: add a larger injector and decrease the injection time to maintain constant mass flow rate. This would give you a smoother-running engine with the same power output.
Well, I found that interesting anyways.
Of particular interest is the autoignition temperature of the fuel. Wentzel points out that heating the cylinder air charge (end gas) to temperatures above about 350*F is "of no interest", since by that point the fuel droplets have almost completely vaporized. Significantly, the autoignition temperature of the resulting charge is much less than 300*F. What's my point? The point is, at the point of maximum compression the autoignition temperature of the mixture is remarkably low.
Also of interest are the results of Wentzels equations for rate of droplet vaporization. They indicate that at TDC, fuel droplets are fully vaporized within 0. 00106 second, even if ignition does not take place. If ignition does take place, the droplet temperature reaches 300*F within 0. 00055 second (recall that we need far less than 300*F for autoignition to occur). In essence, Wentzel is trying to quantify ignition lag. Of course, actual ignition lag is quite a bit longer, since there are some other factors that also come into play; ignition lag is typically on the order 0. 007 seconds. Why the discrepancy? Some of it can be explained by "chemical lag", which is just a natural finite time for the chemical reaction to occur. It is interesting to note that bomb experiments indicate little or no relationship between ignition lag and injection pressure.
Where does the remainder of the lag come from? It seems a lot of it comes from the fact that new fuel being introduced has a cooling effect at the spray core, which reduces the fuel temperature below the optimal vaporization and autoignition temperatures. Much of the lag we see in the Diesel, then, can be attributed to the heat transfer rate between the end-gas and the fuel spray.
So where does this bring us? Well, Wentzel found that detonation is most likely to occur when the injection period exceeds the ignition lag. Translation: injection time must be kept as short as possible. How do we decrease injection time? Increasing fuel pressure won't work, because our injector is an orifice. In an orifice, mass flow rate is independent of upstream pressure until upstream pressure falls below about 1/2 of ambient pressure (pressure in the cylinder during injection). So increasing fuel pressure will not result in any additional fuel entering the cylinders.
Then the only way to do it is to increase the orifice diameter: add a larger injector and decrease the injection time to maintain constant mass flow rate. This would give you a smoother-running engine with the same power output.
Well, I found that interesting anyways.
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