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Direct Injection: More For Less?

3 min read

Direct injection engines have pros like improved performance and better fuel economy. Find out more about how else they affect your car.

From a 2003 Alfa Romeo JTS. It was first introduced in 2002.

In petrol cars, Direct Injection (DI) engines promise power with fuel efficiency. An understanding of how a petrol engine works would shed some light. The three elements needed for how a petrol engine works are air, fuel, and fire. The air is compressed, mixed with the fuel, in the combustion chamber (above the piston heads), where fire (spark) is introduced to ignite the air-fuel mixture. The internal explosion (combustion) pushes the piston down, which turns the drive shaft that spins the wheels.

With DI, the focus is specifically on improving the combustion process. Therefore, DI spark plugs are placed directly above the combustion chamber. The piston heads are also redesigned and are shaped like cones. This offers less space at the top and less volume in the chamber, allowing for stronger explosion with cleaner burn. Thus, resulting in using less fuel without sacrificing power.

Alfa Romeo JTS – the sparkplug sits above the cone-shaped pistons.

An early adopter, 2002 saw Alfa Romeo introduce its 156 JTS (DI). For an apple-to-apple comparison, the same car model will be used, the Alfa Romeo 916 Spider, down to even having the same 2-litre engine block. The only difference, one engine is powered by the Twin Spark design (also promoting clean burn), which was replaced by the JTS (DI).

2003 Twin Spark
Maximum Power (DIN): 150bhp @ 6300rpm
Maximum Torque: 181Nm @ 3800rpm
Fuel Consumption (combined): 9.2l/100km
2003 JTS (DI)
Maximum Power (DIN): 165bhp @ 6400rpms
Maximum Torque: 206Nm @3250rpm
Fuel Consumption (combined): 9.2l/100km

The gains of 15bhp and 25Nm, while fuel consumption remaining equal, were significant for a standard aspirated petrol engine, although it may seem arguably small.

Inherent in its design, DI engines have issues mostly with carbon build-up, and a little engine oil seepage. Noticeably during cold starts and hush or sudden acceleration. Some common symptoms, but not exclusive, are:

  1. increased fuel consumption
  2. misfires
  3. sluggish engine response
  4. faulty air sensor
  5. clogged injectors
  6. black smoke

The above symptoms may occur only after the engine has aged a little. Nothing dire, just follow the manufacturer’s guidelines for service intervals, including using the recommended engine oil and petrol grade. Additionally, check engine oil levels and oil condition regulary.

City driving, unfortunately, equates to higher carbon build-up. Ironically, while at low Revolutions Per Minute (RPM), fuel efficiency for a DI engine is at its best, giving the most fuel economy. However, carbon build-up is also at its highest.

Should symptoms arise, plan drives above 30 minutes (no stops), and keep steady above 2500 to 3000 RPM. In addition, periodically revving near red-line, safely and conservatively, helps. Essentially, build up heat, air and fuel, to flow continuously over the main carbon build-up areas, to remove carbon particles. There are also after-market products that can clear carbon build-up.

Enjoy the benefits of a DI engine, but like all engines, proper maintenance is key to have problem-free motoring.