Volume II, Issue 10, Page 2

The secret’s in the burn...

It seems that much of the technical material you in find print media concerning power increases deals with air flow; e.g., more, better distribution, improved quality, etc.  In reality, in addition to its obvious contributions toward making power, air is the conveyance by which fuel is supplied to the combustion space. There is a fundamental requirement, that at the time of combustion, liquid fuel needs to be as finely atomized as possible.  The core reason for this is simple.

Atomization and combustion efficiency - Combustion of a hydrocarbon fuel (the oxidation reduction reaction process, if you’re a chemistry buff) is time-based. It’s also a function of fuel droplet size.  The larger the droplet, the more time required to convert it into heat or useable work. Since a typical “burn time” is measured in milliseconds, and even less as rpm increases, comparatively large droplets may not have sufficient time to be completely converted into power. The results are wasted fuel, lost power, and elevated exhaust emissions.  Good atomization efficiency is key to optimizing combustion efficiency.

Compared to fuel injection, carburetors are little more than a compromise to atomization efficiency and in some circles they are labeled “controlled leaks.” From a practical standpoint, variances in fuel droplet size amount to differences in air/fuel ratios in terms of how long the combustion process occurs. In effect, large droplets act like “rich” mixtures and smaller ones more like “lean” mixtures. Accordingly, rich mixtures combust slower than leaner ones and this translates into combustion flame travel that’s neither consistent nor uniform as it affects burn rate or cylinder pressure rise.  Alone, this condition prevents optimization of ignition spark timing and fuel metering calibrations, particularly for carbureted engines.

Poor mixing and uncontrolled combustion – When an engine is experiencing poor fuel atomization, the combustion space is populated by a blend of “rich” and “lean” air/fuel mixtures.  At any given point in the combustion process, this blend can vary between cylinders and in the same cylinder on a cycle-to-cycle basis.  In both cases, it’s virtually impossible to optimize spark timing because uncontrolled combustion (detonation) can develop at any time in engine speed and load range. As a result, spark timing becomes a compromise (usually less than optimum) in order to address times when air/fuel ratios are effectively leaner than richer…again based on improper atomization efficiency.

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There are, as you might expect, other factors at play. Variations in engine operating temperature, rpm, changing dynamics in the induction system (we’re dealing with non-steady-state air and fuel flow) and related conditions affecting air/fuel mixture homogeneity can individually and collectively detract from good combustion efficiency. 
The advent and widespread use of electronic fuel injection (especially for common-rail diesel engine EFI systems) has created dramatic improvements in fuel atomization efficiency.  Early on, TBI systems did a better job of delivering increased fuel atomization (compared to carburetors) but there was still the problem of transitioning air/fuel mixtures through the intake manifold’s circuitous passages and brought back some of the air/fuel separation problems associated with carburetors. MPFI systems were a clear improvement over TBI, but there was still the issue of conveying wet flow into the combustion space while minimizing air/fuel separation or altered mixture ratios at the time of combustion.  The current trend in direct-injection gasoline engines is yet another step to solve the same problem discussed a few paragraphs back.  All of these latter technologies have allowed higher mechanical compression ratios for boosted power, even with unleaded gasoline. Uniformity of fuel particle size begets smoother flame travel and reduced tendency toward detonation.

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