This chart is a generalization of in-cylinder combustion temperatures showing the effects
In order to extract the greatest amount of heat from a given volume of fuel, several critical elements come into play. Some of these objectives have been discussed in prior Enginology columns. Obviously, efficient mixing of air and fuel is important. Making certain fuel droplet size (certainly during combustion) as small and uniform becomes essential. Providing sufficient time to complete the burn is linked not only to spark timing but relates directly to burn rate. So because it's a vital feature that can contribute to combustion efficiency and net power, we'll focus this month's discussion on factors involved in burn rate.
Since most racing engines operate at fairly high mechanical compression ratios, air/fuel charge density is high, which helps accelerate the burn. However, proper mixture conditioning as provided by some form of turbulence in the combustion space further aids burn rate, whether by some form of swirl, tumble, or combinations of these.
As explored many years ago by Sir Harry Ricardo and continued into the late '70s by Switzerland's Michael May, along with other students of combustion, the fast burn concept is particularly applicable to circle track engines. Ricardo Engineering also developed a "high ratio compact chamber" (HRCC) concept as an outgrowth of Harry Ricardo's investigations in the '40s. So, the notion of benefits derived from increasing turbulence in the combustion space has been around for a time and simply undergoing further exploration and refinement.
Other consequences of an accel-erated burn include a reduction in exhaust gas temperatures. In fact, if the combustion process is more rapid, it stands to reason more heat will be converted into useable power (before the exhaust event begins), compared to engines with a slower burn rate. Most importantly, it's necessary to work toward improving cylinder-to-cylinder air/fuel charge distribution. Failure to address this concern can lead to lean mixture misfire and periodic or sustained detonation that varies among cylinders operating at or near their lean or knock limits. While it's important to be concerned about cylinder-to-cylinder and/or cycle-to-cycle air and fuel distribution under all conditions, it is particularly critical when making engine modifications that increase the rate of combustion.
But as you might expect, with advantages often come disadvantages, although slight and possible to compensate for in this instance. In particular, creating what we'll call "irregular flow" in the inlet stream and combustion space tends to reduce net volumetric efficiency. The trade-off then involves a comparison of potential power lost through reduced v.e. to that obtainable from improved combustion efficiency. Unless the mixture motion created causes a substantial reduction in volumetric efficiency, offsetting gains in power usually dominate. One middle ground between no intention to improve air/fuel charge homogeneity and specific mixture motion generation points to surface texturing of the inlet path and selected areas of the combustion space. Since this has been a prior Enginology topic, we'll stop short of a regurgitation and simply emphasize the importance of considering the potential gains from surface conditioning.
Also previously mentioned but worth bringing to your attention again is how rapid burn rates affect spark ignition timing requirements (or opportunities). By increasing burn rate, spark timing advance can be reduced, leading directly to less negative torque on the crankshaft in the early, pre-TDC stage of the combustion cycle. Bottom line, less spark advance correlates directly to more net (positive) pressure on the crank.
You should also observe a reduction in brake specific fuel consumption. In a sense, this can relate to on-track fuel economy. From a fuel cost standpoint, you'll be making more power from the same amount of fuel with the potential of reducing fuel use for the same amount of power. Results from either perspective can be a plus. So, if you subscribe to the notion that reduced BSFC (on the dyno) equates to improved acceleration rates on the track, the good side of creating increased burn rates makes even more sense.
So how does all this translate into use if you're an engine builder or racer? Let's carve out specific points raised in this discussion and put them to applications. By the way, if you've been following this magazine's Project G.R.E.E.N. since the January issue, we invite you to begin applying some of this column's subject matter to engines using alternative or sustainable fuels. Even though we won't be investigating how the use of sustainable fuels would be affected by using current ways to enhance the use of gasoline, keep this thought in mind. We're still dealing with an internal combustion engine that provides a chemical reaction with a heat-liberating fuel in a way that mechanically converts to horsepower. An engine's crankshaft cares very little about the specific composition of the substance that's creating its rotation.
Now back to the first sentence of the previous paragraph. A major factor involved in creating a rapid combustion process is air/fuel charge conditioning. Of course it's important to achieve high levels of volumetric efficiency and sustain them throughout the engine speed range of most frequent use. That's not meant to be academic. It's a fact. So think about and work on how you choose to do that. Surface texture does matter, from the point of fuel delivery and throughout the combustion space (if you're using a carburetor) and from injector location to the same space, if FI is your choice for induction system. And don't forget to include piston crown surfaces in your considerations. Just because these represent the combustion space floor doesn't exclude them from importance.
Evaluate your fast burn experimentations on the dyno. As you begin to achieve success, look for reductions in exhaust gas temperatures. If you're fortunate enough to be including exhaust gas analysis equipment as part of your engine (or parts) development regimen, watch for reductions in oxides of nitrogen (NOx). In fact, making this type equipment part of your engine dyno measurement process can be a valuable tool, even though you might not otherwise be concerned about emissions. It'll help you evaluate the combustion process and often point to problems that are masking potential gains in power. Check out the emissions equipment being used in CT's Project G.R.E.E.N. for more information or contact Dave Kalen at Sensors, Inc. at 734/429-2100, ext. 216.
Also, experiment with mixture ratios. You may find that less fuel will make the same amount of power, yet another indicator of an increased burn rate. And don't forget about doing a map of initial ignition spark timing. As a suggestion, make repeated WOT power measurements at peak torque rpm, steady state. Perform a series of runs at spark settings below and above what you would normally run on the dyno, not at the track. If you discover less spark timing is required at peak torque, use that value for a full dyno run. Again, if your modifications to increase burn rate are successful, a trend toward (or actual) power gains will indicate success. At the end of the day, all this can amount to difference between the checkered flag and something less.