If Smokey Yunick was still with us today, I suspect he'd be smack in this middle of this issue. He once told me that carburetors are little more than "controlled leak" devices and that electronic fuel injection in everyday circle track engines was an inevitable option that should be explored and developed at a practical level. Before we dig into the subject, let's take a moment to review some combustion properties that play into the benefits of EFI, as compared to carburetors. While we won't address related topics such as system economics and complexity, both of those elements appear becoming of less user concern than in the past.
On multiple occasions in the pages of this magazine, we have attempted to describe the complicated process of converting liquid fuel to heat (power) by combustion. It is the method by which fuel is atomized and the point of introduction into an inlet airstream (or combustion space) that become problematic to an efficient burn. In addition, while the net effect is mixture passage into the combustion space, flow in an induction system or path is elastic, interrupted and loaded with a variety of pres-sure excursions that tend to "un-mix" air/fuel charges. And the farther upstream in an induction system fuel is introduced, the more influential the problem causers become.
In particular, we know that to initiate and sustain a uniform combustion flame rate, it's helpful to have created air/fuel charges consisting of the smallest possible fuel droplet size. And the mix should be as uniform and homogeneous as possible, throughout the combustion space. Simply stated, carburetors have a difficult time favorably contributing to this objective, if for no other reason than they function well upstream of combustion spaces. In a recent conversation with CT Senior Technical Editor, Bob Bolles, it became apparent that maybe some further discussion on the subject might be helpful to CT's readership.
Here's a perspective to consider. At the risk of oversimplification, imagine that large fuel droplets require more time to be consumed in the combustion process. And, conversely, smaller droplets take less time. In a combustion space of poorly mixed air/fuel charges (especially if there's a range of droplet sizes), there will effectively be an accompanying range of air/fuel ratios in this space. Accordingly, when the burn begins, flame rate will not be uniform and, very possibly, an inefficient net burn occurs.
In the early 1970s, I received a call from a man named Jack Priegel. He was the president of Autotronic Controls Corporation in El Paso, Texas. His company was developing a fuel atomizing device using what amounted to an ultrasonic device through which liquid fuel was being broken into droplets reported to be in the sub-20 micron size. Atomized fuel looked like "smoke" once it passed through the device. But he had what he thought was an intake manifold problem because the device was located where a carburetor would have normally been placed. Despite this handicap and the fact fuel was being taken out of suspension by pulsating airflow, his system demonstrated the potential benefits from a dramatic reduction in fuel droplet size. Jack was funding the project by the sales of a product his company devised just to keep his induction system efforts alive. Actually, that product was a multi-spark ignition system. The induction system was discontinued when sales of the product went through the roof and we all know the impact MSD ignitions have had on motorsports. But that's another story.
The point here is that the atomization efficiency provided by electronically-operated fuel injectors, especially when placed in proximity of the combustion space, offer tangible benefits. EFI systems enable more control over many of the rudimentary requirements of efficient internal combustion engine oper-ation, leading to an opportunity for increased power with reduced fuel consumption and (believe it or not), a chance to decrease exhaust emissions.
Of the arguments frequently offered by non-proponents of EFI in racing, one deals with the use of electronics and the possible sensitivity of these components in a harsh racing environment. Any concerns in this particular area are dwindling, as witnessed by some robust racing engines doing well with not only iterations of OEM systems but aftermarket conversion packages that simply replace carburetors with throttle bodies and supporting electronics. Some companies provide multi-point EFI induction systems that locate injectors in closer proximity to combustion spaces to reduce upstream effects of pulsating and circuitous flow paths. Obviously, mechanical fuel injection systems stand in the gap between carburetors and EFI, but there appears to be movement afoot among providers of mechanical systems to blend electronics into some of these packages.
In previous columns, I've also suggested that an engine's brake specific fuel consumption performance typically occurs at or near peak brake torque. Of the factors that affect this condition, one is engine speed and the other is quality of atomized fuel delivered by a carburetor. At speed below peak torque, inlet mean flow velocities are below those required to aid atomization efficiency. This can be verified by evaluating the extent of unburned hydrocarbons (unspent fuel) in the exhaust gas.
Similarly, at higher rpm, the tendency for a mechanical separation of fuel from the air grows, thereby increasing the chances for poorly atomized fuel in the combustion space, verifiable by the same analysis process. Improvements in carburetor main fuel booster design (or modifications) can help the problem but not remove its influence altogether. As a result, combustion efficiency as evidenced by BSFC data typically shows higher numerical values for this data below and above peak torque.
On the other hand, fuel injectors used in an EFI system tend to supply a higher percentage of atomized fuel throughout a broader range of engine speed. Throttle response generally improves, as does on-track power and fuel economy. Reflecting on these conditions, BSFC curves tend to be somewhat flatter and nearer peak torque values above and below this engine speed, further indicating a wider span of improved combustion efficiency. Add to the package an opportunity to electronically vary cam timing and a new perspective can be taken on the benefits of on-track, electronically-controlled circle track engines . . . don't you think?