Diesel engines are renowned for fuel efficiency and torque. They use compression ignition, which means that as the piston squeezes the fuel and air mixture, it ignites without the need for a spark plug. Their advantages stem from the ability to burn with a very lean air-fuel mixture, and their large expansion ratio (valve timing issues aside, the larger the compression ratio, the larger the expansion ratio). Also, because they are able to run very lean, they control power by adjusting fuel flow, not air flow. Typical gasoline powered engines will alter the flow of air into the engine with a throttle valve, and this causes pumping losses. By contrast diesel engines do not use a throttle valve, and so can run efficiently at idle or small loads.
The challenges with diesel stem from how the fuel is added. Without a spark to control when ignition occurs, these engines rely on compression. In almost all engines, compression is fixed (except the new Nissan engine – more on that in another post), so to avoid ignition taking place too early, before the piston is at the top of its travel and best positioned for a power stroke, fuel is sprayed directly into the engine when power is desired. The problem with that is that it is very difficulty to ensure complete combustion, because the fuel doesn’t have time to fully mix with the available air. This causes soot (damaging to human health) and nitrous oxides (aka NOx – producing acid rain and smog).
A typical gasoline powered engine uses a throttle body to control air flow into the engine and port injection (fuel sprayed into the air just before it enters the cylinder) or direct injection (fuel sprayed directly into the cylinder) or, in some cases like new Lexus motors, both. Because the fuel doesn’t burn immediately when entering the cylinder, gas engine designers have authority over how the fuel mixes and when the spark ignites it. This prevents knock and soot.
Better than both
Reviewing the pros and cons of diesel and gas engines, one might despair that combining the two would simply not be possible. The essential difference is that compression ignition must happen when fuel is sprayed in, resulting in poor mixing, and spark ignition must happen at lower compression ratios to avoid detonation (unwanted compression ignition).
Mazda has an ingenious solution. They use very high compression and very lean mixtures – too lean for conventional spark ignition. To provoke compression ignition (and time it for best power delivery and engine durability), they use direct ignition to spray a little fuel near the spark plug – just enough to make the air-fuel surrounding the spark plug rich enough for spark ignition. The resulting increase in heat and pressure inside the combustion chamber causes the remaining lean mixture into compression ignition.
This is an exciting breakthrough because the majority of the fuel in the engine is ignited in an instant. This is better than gasoline engines in which the flame front travels from the spark plug to the remainder of the air-fuel mixture. And this is better than diesel engines in which ignition occurs while the fuel is being sprayed into the combustion chamber.
Because the majority of fuel being burned is lean and well mixed, very little soot and NOx is produced – even without a catalytic converter.
Not just a concept, Mazda’s new engine, dubbed SKYACTIV-X, is being demonstrated in a Mazda3 similar to the one pictured.
As with almost all the blog posts here, I encounter an exciting innovation and wonder, which of the 40 Principles would Genrich Altshuller see? It’s a useful exercise because it is helping me to catalog examples of the 40 Principles. With a deeper understanding of the Principles comes improved ability to apply them to our own challenges.
Let’s look at the contradictions the engineers are facing here:
- Compression ignition – the known way to avoid undesired ignition is to spray fuel in late, resulting in higher emissions.
- Spark ignition – the known way to avoid undesired ignition is to keep compression low, resulting in lower efficiency.
Their eureka moment was, “What if we treat part of the combustion chamber like a gas engine and the rest like a diesel engine?”
This is an implementation of Principle 3 – Local Quality, as the combustion chamber is zoned – with a homogeneous charge throughout except for a small region around the spark plug which is made richer. Principle 5 – Merging is also at play, as elements of both gas and diesel engines are involved.