How BMW Is Using A Fancy Version Of 1970s Honda Engine Tech To Meet Emissions Without Going Hybrid

Automakers in Europe are feeling the pinch as emissions regulations tighten. Ultra-strict Euro 7 regs are set to take effect in the European Union later this year, with manufacturers scrambling to hybridize and electrify their fleets to remain compliant. One such company is BMW, which is currently in the midst of a big electrification push with its line of “Neue Klasse” EVs, including the iX3 crossover and i3 sedan.

While its big-boy M cars like the M5 and XM have also been treated to partial electrification, BMW isn’t giving up on pure gas-powered drivetrains. The company announced today it’s implementing a new version of technology once used by Honda in the 1970s to bring its high-horsepower straight-six up to modern emissions standards.

The tech lies inside the engine itself, specifically within the cylinder head. It’s called combustion pre-chamber ignition, and it promises huge strides in efficiency, specifically when the engine is under high load.

The Revolutionary Tech

Pre-chamber combustion tech has been around for a long time. American audiences got their first taste of the concept in the mid-1970s with the introduction of Honda’s CVCC engine, installed in its Civic hatchback. CVCC stands for Compound Vortex Controlled Combustion, which doesn’t do much to explain the tech. But basically, there’s a chamber in the cylinder head that sits above the main combustion chamber, with its own fuel delivery path and spark plug. This “pre-chamber” ignites its air-fuel mixture first, pushing those flames into the main combustion chamber and causing a vortex that makes the air-fuel mixture there burn a whole lot more evenly.

Here’s a diagram showing the CVCC engine’s combustion cycle:

Honda’s system burned fuel so cleanly that it could pass California’s then-newly minted emissions regulations without the use of a catalytic converter. Interestingly, the testing was done on the engine using a car that wasn’t a Honda at all. From the company’s history file on CVCC:

“Test data was taken with the CVCC engine installed in a Nissan Sunny,” said Ken Mizoguchi, the Honda representative on-site in Ann Arbor. “At that time, Honda still didn’t have a car that was big enough for the CVCC engine. We even used sandbags to try to increase the weight.”

Indeed, the Civic had only been introduced a short time before, on July 21, leaving Honda with no alternative but to take the qualification test with the body of a competing vehicle.

CVCC proved so effective that other automakers couldn’t help but put it in their own cars. Soon after its introduction, Honda signed licensing agreements with Toyota, Ford, Chrysler, and Isuzu to allow the tech to appear in their engines.

The Revolution Continues

In an effort to keep its smaller M cars purely gas-powered in the coming years, BMW is introducing a pre-chamber ignition system of its own to its S58 3.0-liter turbocharged straight-six engine. Found in the M2, M3, and M4, this refreshed powerplant will become standard equipment on those cars starting in July for the M3 and M4, and in August for the M2.

Though the basic concept is the same as Honda’s original idea, there are some big differences. For one, the CVCC setup is a totally passive system, and uses a carburetor to feed the pre-chambers and main cylinders. BMW’s version, on the other hand, is active and adjustable, with the engine’s computer able to activate or deactivate the system based on load and revs. The pre-chamber and the main combustion chamber each get their own spark plugs, meaning there are 12 per engine (have fun servicing this thing, BMW techs).

Here’s how BMW decides when and when not to use the pre-chamber system, per its press release:

At low and medium revs, the conventional spark plug in the main combustion chamber fires before the spark plug in the pre-chamber. But when the engine is operating under higher revs and loads, the pre-chamber ignition takes over the lead role, with part of the fuel-air mixture channelled through the openings into the pre-chamber also ignited there. The flames generated as a result exit the pre-chamber at around the speed of sound.

These ignition jets then ignite the mixture in the main combustion chamber above the piston at multiple points at the same time. The result is a significantly higher combustion speed. Meanwhile, the possible cause of uncontrolled combustion – i.e. “knocking” – is also countered effectively. An additional effect of this technology is a drop in the temperature of the exhaust gas.

BMW has created some nice visuals that depict the difference between a normal combustion cycle and a cycle using the pre-chamber ignition. First, here’s how the engine works without the system active (note the spark plug on the left firing and igniting the air-fuel mixture, with no involvement from the pre-chamber on top):

And here’s how the combustion occurs when the pre-chamber is activated. The initial ignition occurs in the pre-chamber first, before it propagates into the main chamber to ignite the rest of the air-fuel mixture:

Instead of just a singular pathway from the pre-chamber to the main combustion area, like what’s shown on the Honda diagram earlier in this article, BMW has split the pathway into seven separate “jets,” so that the flames spread out more evenly through the main combustion chamber. Here’s a top-down visual to give you a better idea of what that looks like:

In addition to the pre-chamber ignition system, it’s also given the S58 a higher compression and variable-geometry turbochargers to boost efficiency further. The company doesn’t say exactly how much more efficient the engine is, but describes the improvements as dramatic, “especially when the engine is being pushed to its limits.” If you take your M car to the track often, that should mean noticeable savings. From the release:

Under high loads, fuel consumption drops substantially. This is particularly beneficial for BMW M customers who drive their cars on race circuits – e.g. during track days: the fuel consumption reduction enabled by BMW M Ignite technology means they can keep lapping for longer on the same amount of fuel.

This kind of active pre-chamber combustion tech isn’t totally new. Engines like the Nettuno V6 found in the Maserati MC20 and GrandTurismo, as well as the Hurricane 4 inline four-cylinder in the Jeep Grand Cherokee, use similar cylinder head setups. But with emissions regs now tighter than ever, it’s not terribly surprising that more companies like BMW are spending the extra development and manufacturing costs to implement it into their cars.

While pre-chamber combustion systems make engines even more complicated than they already are, I think this is a worthy tradeoff. To me, a few more spark plugs are a better alternative to adding a heavy hybrid system or losing the gas engine altogether to purely electric power.

Top graphic image: BMW