Fuel Atomisation

[Greg Hermann]

At 1:37 AM 12/10/01, Mos wrote:
>On Fri, 7 Dec 2001, Greg Hermann wrote:
>
>> And--the motivating force behind DFI, is, again, regulations. This time
>> mostly CAFE.
>>
>> Speaking of which--DFI is the only thing that can compete with DCOE/IDA
>> carbys in terms of BSFC !!
>> Why?? Because it HAS GOT to have good fuel atomization to even function !
>
>So what you are saying is that carbs (I guess not all) have better
>atomisation than port injection? and consequently lower BSFC? (As a
>generalised statement - feel free to go into much detail).
>
>Mos.

You catch on quickly!!

DCOE's have _such_ good atomization that you can let off the choke about 5
to 10 seconds after a seriously cold start.

Most carbys are not that good, and port injectors are not even close.

Squirting fuel on the back side of a closed intake valve (as some port
injectors do) so as to vaporize it (to make up for the lack of good
atomization) hurts both volumetric AND thermal efficiency.

The trick here is to have as much of the _vaporization_ (as opposed to
atomization) take place _after_ the intake valve has _closed_ .

What this does is let the latent heat of the fuel (as well as of any
injected water, SHHHH!!) act as an internal coolant during the compression
stroke. Internal cooling during the compression stroke reduces the negative
work needed during that stroke, reduces the peak temp of the cycle and the
EGT, reduces the peak pressures during the cycle, and reduces heat
rejection to the water jackets significantly as well.

The temperature/pressure reductions can get pretty significant when you are
working with methanol fuel and/or a significant amount of H2O injection.
And there is _NO_ net loss in power, in fact there is a gain.

Good atomization also leads to more uniform distribution of the fuel
throughout the charge, leading to better, faster combustion (also more
power and efficiency here).

All of which add up to improvements in power, efficiency, as well as
mechanical durability (lower pressures and temps do that!) as a direct
result of better fuel atomization.

Industrial oil burner practice backs up these points--the lowest cost,
efficiency burners use mechanical atomization, next step up is compressed
air atomization, and the highest step is steam atomization. And boilers
don't even get cycle efficiency benefits that Otto cycle engines get from
improvements in fuel atomization!

Greg


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