Why Some 4-Cylinder Engines Use 8 Spark Plugs

Why Some 4-Cylinder Engines Use 8 Spark Plugs

When you pop the hood of most cars, you’ll typically find an equal number of spark plugs and cylinders. A four-cylinder engine, for instance, usually has four spark plugs. Simple, right? Not always! You might be surprised to learn that some four-cylinder engines actually come equipped with a whopping eight spark plugs – two for each cylinder. So, what’s the deal with this seemingly redundant setup? Let’s dive into the fascinating reasons behind this engineering choice.

The Dual-Plug Advantage: Beyond Just Sparking

The primary motivation for employing two spark plugs per cylinder, especially in a four-cylinder configuration, boils down to optimizing the combustion process within the engine. Here’s how it works:

  • Improved Combustion Efficiency: When a single spark plug ignites the air-fuel mixture, the flame front spreads outward from that single point. With two spark plugs, ignition occurs simultaneously at two different points within the combustion chamber. This creates two flame fronts that meet in the middle, significantly speeding up the combustion process. A faster, more complete burn means more of the fuel’s energy is converted into power, rather than being wasted.
  • Reduced Emissions: More complete combustion directly translates to fewer unburnt hydrocarbons and other pollutants being expelled through the exhaust. By burning the fuel more thoroughly, engines with dual spark plugs often meet stricter emissions standards without needing overly complex or restrictive exhaust after-treatment systems.
  • Enhanced Power and Torque: While it might not double the horsepower, the more efficient and rapid burn can lead to a slight but noticeable increase in power output and torque, especially at lower RPMs. The engine can operate more smoothly and respond better to throttle inputs.
  • Greater Reliability (Secondary Benefit): Although not the primary design goal, having two plugs per cylinder can offer a small degree of redundancy. If one spark plug were to fail or foul, the other could potentially still ignite the mixture, allowing the engine to continue running (albeit possibly with reduced performance) until repairs can be made.
  • Design Flexibility: In some engine designs, particularly those with complex cylinder head shapes or lean-burn strategies, two spark plugs can help ensure proper ignition across a wider range of operating conditions.

Who Uses This Technology?

While not a universal practice, several manufacturers have utilized dual spark plug technology in their engines over the years. Honda’s i-DSI (intelligent Dual Sequential Ignition) engines, found in models like the Fit/Jazz and City, are prime examples. These engines often prioritize fuel economy and lower emissions, which the dual-plug system helps achieve. Other brands, notably Alfa Romeo with their ‘Twin Spark’ engines, have also famously employed this setup, often with a focus on performance and responsiveness.

The Bottom Line

So, the next time you hear about a four-cylinder engine with eight spark plugs, remember it’s not a mistake or an oddity. It’s a clever engineering solution designed to squeeze every last bit of efficiency, power, and cleanliness out of the combustion process. It’s a testament to the continuous innovation in automotive technology, always striving for better performance and a greener footprint.

Source: Original Article