In a typical four-stroke cycle engine, it is virtually impossible to avoid some form of contamination in otherwise combustible air/fuel charges. But before we dig into the effects of such contamination, let’s look at the conditions that can cause it in fresh mixtures.
First, pressure conditions in the inlet and exhaust paths and corresponding pressures in the combustion space, throughout all four cycles. To make this as simple as possible, our example engine will be a single-cylinder design.
The reversion process begins at the opening of the intake valve. At this point, the exhaust valve is open and has usually been allowing combustion residue to exit the engine. When the intake valve begins to open, pressure in the intake manifold in a normally aspirated engine is at or below atmospheric—generally, below—and residual exhaust gas begins to flow back into the intake manifold. This condition continues until the pressure in the intake manifold is briefly equal to cylinder pressure. Even though the intake cycle begins before the piston reaches BDC on the exhaust stroke (considerably before with racing engines), a certain amount of incombustible gas has already entered the intake track.
As a function of revolutions per minute and when the intake valve begins to open, efficiency of the exhaust system and related variables, the time and distance traveled into the intake manifold, and volume of contaminating gases varies. Generally, the higher the engine speed, the less time for this transfer to occur, but it happens nonetheless. I’ve seen engines running at relatively low revolutions per minute and wide open throttle display a cloud of vaporized fuel above a carburetor’s air horn. As engine revolutions per minute increases, this cloud is forced into the engine due to the reduction in time for the so-called standoff cloud to form. Intake manifolds that employ some type of plenum volume tend to minimize this condition, when compared to individual runner (IR) manifolds that typically do not use plenums or some method to dampen pulses.
As revolutions per minute increases, reversion contaminates the combustion process. I’ve seen reversion cases sufficiently serious it stains the bottom of the carburetor throttle plate with exhaust gas. The stronger the reversion pulse, the greater the amount of energy needed to equalize intake manifold and cylinder pressures to allow the intake cycle to begin.
At some moment in the reversion process, pressure in the intake path and cylinder equalize. With the exhaust valve closed, the true intake cycle begins and allows atmospheric pressure to force and fuel into the cylinder. Eventually, the intake valve closes, the mixture compresses and ignites, and the entire process repeats.
Take what occurs in a single-cylinder engine, and visualize connecting seven more single-cylinder intake manifold runners to a common volume (plenum). Depending on the engine’s firing order (let’s say its 1, 8, 4, 3, 6, 5, 7, 2) and the fact all runners usually share the same reversion cycles and periods (unless something has been changed in the camshaft’s design to address the problem), you can see there are noncomparable pressure conditions among the runners. For example, even though they are side-by-side in their union with the manifold’s plenum, runners 4 and 2 are separated by several crank degrees, as compared to cylinders 5 and 7 that do not enjoy that separation. As a result, among other related issues, air/fuel mixture distribution between the latter two cylinders is often a problem with this firing order when using a single-plane manifold. Note that cylinder 7 begins its reversion and contamination period during the intake stroke for cylinder 5.
It’s not just that the reversion/contamination situation is isolated to individual cylinders. This so-called cross talk among cylinders joined through the plenum chamber can have a negative impact on combustion efficiency and power. The problem is somewhat comparable to the short-runner/long-runner design in these types of intake manifolds. The conditions of other cylinders can affect the performance of any one cylinder. The bottom-line, there is two-directional flow in a typical inlet path and the material passing in either direction, toward or away, from the cylinders isn’t always combustible or contamination-free.
Even if we attempt to rid the combustion space of contaminants, more than likely, when the combustion process begins there will still be some incombustible material in this space. And it will have displaced any fresh air/fuel charges that are combustible. Not unlike the introduction of recirculated exhaust gas (EGT) for the reduction of NOx (emissions reduction), residual exhaust gas in the combustion space will reduce net heat produced, thereby, causing a decrease in power and on-track fuel economy.
Yes, extreme reversion can create a fuel standoff condition, particularly in a single-plane intake manifold. Actually, the same condition could potentially exist in fully divided two-plane designs, except for the dampening effect provided by this type design.
How can you identify signs of reversion and address the problem? Here are a few of the more common ones. Inspect the areas around the cylinder head and manifold surfaces. Traces of exhaust gas coloring in such locations indicate reversion. Check for such traces on the long side of the manifold’s runners, particularly in single-manifold designs. The underside of carburetor throttle plates is another indicator. In addition, a rapid increase in brake specific fuel consumption (BSFC) numbers above peak torque revolutions per minute and reduced combustion efficiency is likely being affected by reversion (contamination). Under such conditions above peak torque, you may see a reduction in EGTs below what you’d expect at higher rpm power levels. Exhaust gas residue tends to reduce combustion temperatures.
Interestingly, you can address a few things in the cylinder heads to reduce the reversion. One, improve low-lift flow in the exhaust port. A reduction in backpressure and a re-shaping of exhaust valve seats, pockets, and ports, can help. Also work on a reduction in flow direction back toward the combustion space. Valve seat and head modifications that help keep flow away from the combustion space and not travelling in a back-flow direction can address this issue. To evaluate such modifications, you can reverse-flow intake and exhaust ports (on an airflow bench) and determine how effective your modifications have been for the low-lift flow. By increasing low-lift exhaust port flow, you will be trending toward lower backpressure when the intake valve first begins to open. This will reduce the strength of reversion spikes at this point in the intake cycle and net potential for increases in volumetric efficiency (torque).
See the little sketch of a time-pressure trace that chronicles the positive and negative pressure history during one full intake cycle. It represents a measurement taken approximately at the junction between the cylinder head and intake manifold on a running engine. This is what you’d see in a single-cylinder engine. You’ll recall the complexities introduced with a multi-cylinder engine using a single-plane intake manifold. Regardless, the illustration here sheds light on the actual pressure excursions.
The amplitude and duration of the initial spike represent the reversion period. The area bounded by the horizontal axis and the pressure trace represents the filling or volumetric efficiency of the engine. Finally, the little spike at the end of the cycle amounts to the decay of the energy once the intake valve closes. Any successful attempt to reduce the amplitude or duration of the reversion spike usually nets a power increase (increased volumetric efficiency).
Any changes in air flow pressure differentials or pulses created across a carburetor can alter the amount of fuel delivered to an engine and upset the desired carburetor calibrations. It’s best to identify the presence of reversion and take steps to reduce its effects on your engine power.