If you often build, rebuild, or are just plain curious about optimizing the power from your engine, this subject is required reading—and critical to producing race-winning horsepower. Simply stated, it’s just that simple.
Here’s the deal. By improving upon the homogenization (mixing efficiency) of fuel and air prior to and during combustion, it’s possible to increase flame speed. As the speed of combustion is increased, the amount of ignition-spark timing (before top-dead-center, or TDC, piston position) required to make the burn can be decreased. Fast-burn engines like reduced spark timing.
The later the point of ignition, the less work (pressure) is applied against the piston’s motion approaching TDC during the compression stroke (if flame speed could be increased to the point of instant complete burn, the ideal would be to introduce the spark at the moment of TDC). And as this “negative” work is reduced, the net amount of work (pressure) applied to the piston as it moves down during the power stroke is increased. If you equate this pressure (before and after TDC) to torque, which you can, it should not be difficult to recognize the benefits of increased flame rate.
Of the ways mixing efficiency can be improved, so-called mixture motion is currently undergoing study and development by some of the finest engine designers and builders in the business—including the OEM motorsports community. This story explores one facet of mixture motion as applied to the layout of or modification to piston “crowns” or tops.
In Its Earlier Stages
Sir Harry R. Ricardo made certain statements about turbulence in his academically classic High Speed Internal Combustion Engine books of the early 1900s. Among them was the following: “The rate of burning depends primarily upon the degree of turbulence and may be expressed in terms of increase of pressure per degree of crank angle.” This approach is currently used in the method of Engine Cycle Analysis (ECA) being employed by leading Winston Cup teams. His statement clearly defines the fact that mixture completeness (homogeneity) and burn rate are related.
Ricardo also noted that the need for turbulence decreases in proportion to an increase in mechanical compression ratio (c.r.). Stated another way, as compression ratio is lowered, the need for and benefits from mixture motion increase.
For example, gains from turbulence in a 9:1 c.r. engine gasoline will be more substantial than in one of 13:1 c.r., although some gains are possible at the higher c.r. In addition, Ricardo suggested that a turbulent vs. non- turbulent engine tends to experience detonation earlier by comparison. This provides an opportunity to remove a measure of spark timing, allowing for an increase in net cylinder pressure after TDC and subsequent gain in torque (positive vs. negative work).
Basic In-Cylinder Mixture Motion
Of the types of motion popularly being studied, swirl and tumble are foremost. Motion that tends to be directed along the plane of inlet flow generally describes swirl. This can be movement in the same direction as this flow, or in an opposite direction. Positive swirl is motion in the same direction as incoming flow. Negative swirl is in an opposite direction but still in the plane of the inlet flow. Tumble is motion akin to rolling a ball down a flight of stairs—essentially end-over-end flow in a plane roughly perpendicular to the swirl plane or axis of the crankshaft.
The shape of a piston’s crown can affect both swirl and tumble. In its simplest form, a flat-top piston can influence swirl more than tumble. This is because protrusions above the piston’s deck surface are minimized, leaving a broad surface upon which features can be built for swirl enhancement. These include tapered ramps and/or strategically placed texturing or dimples, all intended to modify the quality of swirl, improve quench, and aid combustion efficiency.
Where engine-building rules permit higher mechanical compression ratios, the effectiveness of piston-crown protrusions in the combustion chamber should be carefully considered. Protrusions, or shapes built into the cylinder head, can encourage flame migration toward the exhaust valve, but at the same time they increase compression ratio. This opens the possibility that such protrusions can cause an impediment to flame movement and a disruption of the air/fuel mixture during the burn. The potential result is lost power and possible detonation from excessively lean blends in affected areas.
When considering the advantages of manipulating the shape and texture of the piston crown to enhance in-cylinder mixture motion, remember that there are advantages and disadvantages to swirl and tumble. Even though swirl tends to increase flame rate and help deliver a more uniform rate of combustion, used improperly it can lead to increased spark timing and reduced volumetric effi- ciency. Excessive swirl can also cause heavier fuel droplets to be centrifuged from the airstream, which disrupts the in-cylinder air/fuel mixture ratios. Tumble also has advantages and disadvantages. Much like swirl, it can aid combustion rate and uniformity. It can also reduce net volumetric efficiency, leading to losses in net torque. But implemented correctly, either by itself or in conjunction with swirl, tumble can produce increased power.
Today, the instrumentation technol-ogy that allows swirl and tumble to be measured is becoming increasingly affordable to engine builders and designers. Once priced in a range typical of laboratory-grade equipment, swirl-and-tumble meters are now being marketed for aftermarket and enthusiast use.
Understanding Piston-Crown Burn Patterns
Unfortunately, this is not an exact science. Experience and the correlation of visual inspection with engine dynamometer data (power and brake- specific fuel-consumption information) and on-track performance is currently of high value. However, there are some Saturday-night rules of thumb worth mentioning.
Uniformity of color is one area to consider. We’d like for the air/fuel mixture to be relatively uniform throughout the combustion space (in reality, this is difficult at best); a uniform burn-residue color across the face of the piston is the target. Variations in combustion residue point to different air/fuel ratios that existed during the burn. Dark areas suggest fuel-rich burns, while lighter patterns indicate more air-rich mixtures were involved. Portions not showing color usually indicate a lack of combustion or periods of exposure too brief to leave any residue. These also can point to problems of fuel wash or an overabundance of fuel to prevent complete combustion.
Keep in mind that mechanical compression ratio alone does not always lead to detonation. Poor mixture quality (a wide range of air/fuel ratios within the combustion space) often causes detonation. The ability to achieve and sustain high cylinder pressure must include well-mixed air/fuel charges in order to prevent or limit detonation.
If the engine you’re tuning likes spark advance, does not live up to torque-output expectations, requires more fuel than you think necessary, and is detonation-prone, it’s a good idea to examine your piston crowns. Each (and all) of these symptoms often is an indication of poor mixture quality, typified by excessively lean and rich regions within the combustion space. This is often the result of a separation of the air and fuel. So spend some time reading the piston tops; it’s just another tool for hearing what your engine is telling you. Note the accompanying photos for some examples of combustion problems.
It is also possible that unequal cylinder-to-cylinder air quantity and quality can cause torque output variations beyween cylinders. Just because all cylinders are receiving equal (or near equal) amounts of air and fuel, the quality of the mixture in each cylinder can vary—creating unequal cylinder-to-cylinder torque output. Reading the tops of the pistons is a quick method of determining whether you have different mixture properties in different cylinders. Adjusting individual cylinder spark timing helps, but unless or until you are able to equalize the burn process among the cylinders, you’re still dealing with multiple engines of unequal and unoptimized power.
How Can You Tell if It’s Working?
It bears repeating that improved mixture quality tends to accelerate flame rate. As a result, it creates some conditions that do not require sophisticated measurement techniques. By producing an air/fuel blend with smaller droplets of a more uniform size, flame travel will be both smoother and quicker. This conditioning also includes the reduction of puddling, which results from mechanical separation of air and fuel.
Here is an example: If flame speed is increased, less initial spark timing is required to complete the combustion process. The later the spark, the less pressure will be exerted on the piston prior to TDC on the compression stroke. So, if you are working on the objective of improved mixture quality and discover less spark timing is required for optimum on-track performance, you’re on the right path.
There’s also a chance you’ll dis-cover an improvement in fuel econ- omy. Even if mandatory pit stops aren’t on the agenda, saving some money on the cost of current fuels can help—certainly over the length of a racing season. Also, see if you can take some jet out of the engine or simply reduce net fuel flow. Generally, if mixture homogeneity is improved, less fuel is required to make the same (or more) power as before. Again, this can represent a fuel savings. At the same time, throttle response and off-the-corner torque tends to be sharper. You will find the engine accelerates through the rpm range more effortlessly. Stated another way, “transient torque” improves.
If you’re measuring exhaust gas temperatures, faster burn rates usually translate into lower exhaust gas temperatures. The net effect of this is that more heat (work) is released within the combustion space. Therefore, temperature in the cylinder will be lower at the time of exhaust valve opening.
In Summary
Mixture (or air) motion in the combustion space is affected by several variables. Racing rules don’t always permit modifying or using parts to improve this motion. In the best case, you’d like to begin flow-quality improvement early in the inlet path and continue it all the way through the combustion process.
However, if you’re allowed to make changes to the piston crown configuration, specific benefits can be derived. Piston-head shapes that improve flame speed are also a good way to maintain—and sometimes even improve—available horsepower, even as sanctioning bodies mandate ever lower compression ratios. But even if rules prevent you from making any changes at all, the ability to read the combustion chamber floor is a positive step toward finding ways to optimize engine performance.
