Igniting Lean Mixtures

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Flame-jet ignition (here one from Mahle Powertrain) can allow for a quicker and more complete burn of leaner mixtures. (Mahle Powertrain/)

Editor’s note: If you find this topic interesting, check out Bruno dePrato’s recent story on Aprilia’s Ditech fuel-injected two-stroke engine. Part of that concept was to use an extra-lean stratified charge, as Kevin Cameron mentions in this piece. Find the Aprilia story here.

When an engine’s fuel-air mixture is leaned out beyond 18:1, spark ignition becomes chancy; the same is true on the rich side of a 10:1 mixture. Any reasonably instrumented engine lab can verify these limits, yet there is a powerful reason to want to run engines on mixtures leaner than 18:1: improved fuel economy.

You may ask, as I did, “Isn’t the power produced by combustion in simple proportion to the amount of fuel burned?” No, it’s not that straightforward. The uncomplicated explanation for how combustion heat is transformed into mechanical power goes something like this: Fast-moving molecules of hot combustion gas, incessantly colliding with each other and banging into the piston crown, add up to the pressure that turns the crankshaft.

In reality, it turns out that not all of the combustion heat energy goes into the speed of whizzing gas molecules. The hotter the gas, the greater the fraction of its energy that becomes molecular rotation or vibration—energy forms not available to push pistons.

Related: Turbulent Jet Ignition Comes to Formula One

Yet another source of energy loss at higher temperature is molecular dissociation, especially of carbon dioxide. Dissociation occurs because of the statistical nature of energy distribution in gases, when the occasional molecule receives enough energy to break it apart. Since dissociation requires energy, unless that energy is recovered by reuniting the fragments in the combustion chamber, it’s lost out the exhaust ports. Even if dissociation is reversed in the cylinder, the reunification may occur too late in the stroke to contribute to peak pressure.

This tells us why we can achieve greater economy with leaner-than-chemically-correct mixtures: The cooler the combustion, the less released heat energy is lost into the unavailable states described above. With more of the fuel’s potential heat energy going into molecules beating on piston crowns, fuel economy improves.

And this leaves us with the question of how to ignite such lean mixtures to realize improved fuel economy. For a time there was enthusiasm for superexotic igniters such as lasers, plasma injectors, and other Star Wars-sounding technologies. The two that have proven practicable are stratified charge and turbulent flame jet.

Stratified-Charge and Flame-Jet Engines

In a stratified-charge engine the mixture as a whole is lean, but by various means a small zone near the spark is made rich enough to ignite.

In a flame-jet engine a small pre-chamber contains a fuel injector and a spark gap. At the point of ignition, a bit of fuel is injected into the pre-chamber, which the spark ignites. This produces multiple flame jets projecting through radial ports. The jets have vastly more igniting energy than any spark. Plus, since they project from the central injector out to the cylinder wall like spokes of a wheel, they produce a rapid light-up of the lean main fuel-air charge.

Bear in mind that excessively lean or rich mixtures burn more slowly than does a best-power mixture. This is because the presence of excess air or excess fuel exerts a cooling effect on the flame, slowing its progress.

Flame-Jet Engines in Formula 1

While continuing my reading of Calum E. Douglas’ The Secret Horsepower Race (about the hectic technology battle fought during World War II by Allied and German aircraft engine designers), I learned of a clever German answer to this problem of igniting lean mixtures: just squirt in a tiny bit of special fuel having a low auto-ignition threshold (some ethers have this property). The mixture then auto-ignites in diesel fashion from the heat of compression and generates a flame big and energetic enough to light up a cylinder filled with weak mixture.

Because Mr. Douglas has given lectures on advanced aircraft piston-engine technologies to F1 teams, it occurs to me that we may see his hand in the recent appearance of flame-jet ignition in Mercedes and perhaps Ferrari F1 engines. In this technology, spark plug and fuel injector are combined into a single unit located at the center of the combustion chamber. The part’s first job is to supply fuel to the air charge entering the cylinder on the intake stroke. Later, with the piston nearing top dead center, it provides ignition as well: Fuel is added to a pre-chamber in the plug and a spark ignites it. As described above, the resulting flame expands through radial ports to ignite the mixture in the combustion chamber.

This process is of special interest for exceptionally oversquare engines, in which the bore is much greater than the stroke. In such engines the combustion chambers are vertically thin and radially large, and consequently, their charge burns slowly because they are poor storehouses for the high velocity of the entering air charge. Such slow combustion in the high-revving racing engines we find in F1 and MotoGP has required very early ignition, in some cases as early as 60 degrees BTDC. Exposing piston crown and combustion chamber surfaces to flame temperature for so long has led to considerable power loss. Turbulent flame jet ignition presents a possible solution.

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