Automakers offering combustion and hybrid vehicles need to continuously innovate to meet ever-evolving fuel economy and emissions standards.
In today’s market, that often means looking for opportunities to make incremental gains in powertrain performance: whether it be finding ways to reduce the mass of an engine, better leverage the efficiency opportunities of a hybrid powertrain, or improve the powertrain’s software and electronics layer.
One area where automakers have made significant advances in recent years is the development of ultra-lean burn technology.
In the 19th century, the German engineer Nicolaus Otto laid the foundation of the modern gasoline internal combustion engine used in most automobiles.
While the exact engineering has changed over the past 150 years, the central logic of what happened in an engine cylinder has remained largely the same since. While some permutations do exist, the eponymous Otto Cycle pioneered by Nicolaus is still the basic blueprint for most non-diesel automotive engines.
The Otto Cycle describes the four steps of what happens in an engine’s cylinder during burns:
From the mid-twentieth century onwards, engineers and automakers began to explore ways to better control the ratio of air and fuel that enter an engine cylinder during the intake stroke. They looked for ways to increase the ratio of air to fuel required during an intake stroke – thus reducing the amount of fuel that was needed during a burn cycle.
By manipulating the timing of the intake stroke itself and the act of injecting fuel into the engine cylinder, along with using technologies like turbochargers to force more air inside, many vehicles now make use of so-called lean burns with slightly heightened air-to-fuel mixes. In exchange for a slight reduction in torque, lean burns can significantly reduce the amount of fuel needed by an engine – making them ideal for use when a vehicle is already in motion.
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Lean burns do come with trade-offs. Along with a reduction in torque – which is particularly needed when a vehicle is starting up and accelerating – lean burns also can cause an engine to overheat, and promote the overproduction of harmful nitrous oxides. This often means that frequent, leaner burns demand complex thermal management solutions, catalytic converters, and emissions control systems.
However, one of the main structural ceilings to achieving higher air-fuel ratios – or running lean burns for longer – lies in properly controlling the behavior of the ignition event itself. In particular:
There’s a significant risk of engine “knocking” from spontaneous combustion events, which are caused by higher air temperatures causing fuel to ignite out of sync with the engine cycle.
The highly dispersed fuel may not evenly burn following spark ignition, resulting in reduced efficiency and wasted fuel.
The eponymous Otto Cycle pioneered by Nicolaus Otto is still the basic blueprint for most non-diesel automotive engines
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Innovations in lean burn technology continue to address the challenges of operating with increasingly leaner air-fuel mixtures. Advances in ignition technology, fuel injection and combustion-chamber design are helping engineers control how fuel is distributed and ignited within the cylinder, enabling more complete and stable combustion even at higher air-fuel ratios.
For example, one approach is to create different fuel concentration levels within the combustion chamber, concentrating fuel around the ignition site while maintaining a much leaner mixture elsewhere in the cylinder. This ensures the initial ignition event seeds a complete burn throughout the engine cylinder, preventing the risk of wasted fuel starving out ignition cycles.
At Horse Powertrain, we are continually researching and developing the next generation of combustion technologies – with one of our major technology focuses on delivering ultra-lean burn capabilities. With these solutions, we can allow automakers and consumers to enjoy a new generation of reliable cars with ultra-high efficiency and reduced emissions – whether that be greenhouse gasses or nitrous oxides.
Find out more about our solutions here.