What is direct injection? What are the benefits and downsides of direct injection?
While direct injection was previously more common in diesel engines, gasoline direct injection (GDI or DI) has taken the automotive industry by storm over the last decade. Although BMW used direct injection in airplane engines in the 1930's they did not implement DI until the mid 2000's. BMW's N54 engine, introduced in the 2007 335i, was the first engine to feature direct injection and turbocharging. Looking at the current BMW line-up, all gasoline engines available in the US are now turbocharged and direct injected. BMW made the switch to direct injection for good reason, however it also comes with some downsides.
What is BMW Direct Injection?
Prior to the switch to direct injection, most BMW engines were powered by port injection. Direct and port injection are similar in many facets. Both injection systems utilize computer-controlled fuel injectors to spray fuel into the cylinders; the difference lies wherein fuel sprays into the engine.
- Direct Injection - Fuel sprayed DIRECTLY into the cylinder
- Port Injection - Fuel sprayed into intake PORTS
The names may make a bit more sense now. Direct injection sprays fuel directly into the cylinder while port injection sprays fuel into the intake ports. Right now this topic may sound pointless. Both direct and port injection accomplish the same goal of spraying fuel into the engine. Why does it matter specifically where the fuel sprays into the cylinder? Are there actually significant benefits and downsides? Great questions.
BMW Direct Injection
It is hard to believe there can be any differences worth mentioning. However, direct injection has many benefits as compared to port injection, such as:
Benefits of BMW Direct Injection
- Improved fuel efficiency
- Lower emissions
- Better performance
- Cooler cylinder temperatures
Direct injection improves fuel economy by as much as 10-15%, and may lower emissions by a similar amount. Fuel spraying directly into the cylinder can be measured more accurately as compared to port injection. Accurate fuel injection also enables BMW engines to take advantage of lean-burn (more air relative to fuel). Additionally, the fuel atomizes and ignites quicker leading to a more efficient burn. In other words, fuel ignites almost instantaneously rather than taking time to travel through the intake ports and into the cylinder. All in all, this leads to improved performance and cooler cylinder temperatures. Despite the benefits of direct injection, there are also considerable downsides to BMW direct injection.
Downsides of BMW Direct Injection
- Costly injectors
- Need for high pressure fuel pumps (HPFP)
- Lack of fuel flow
- Carbon build-up on intake valves
BMW's already have demanding maintenance and expensive repair costs. The last thing you want is an issue with the direct injection system. DI utilizes extremely high pressure to improve it's benefits. Fuel is often sprayed in excess of 1,500 PSI - compared to port injections 30-60 PSI. This generates the need for expensive HPFP's. If you are familiar with the BMW N54, you likely hear or have heard about the early issues with the high pressure fuel pumps. The early HPFP issues took BMW nearly 5 years to resolve.
Additionally, the tips of direct injectors sit inside the cylinder and are exposed to extreme heat. Between excessive heat and demanding pressures, the fuel injectors must be made from high quality materials. We replaced all six direct injectors on our 2007 335i for a cool $1,500; a repair that likely costs 1/3 of that on a port injected BMW engine.
BMW Carbon Build-Up/Walnut Blasting
Engines naturally create oil blow by that travels through the intake valves. As port-injection sprays fuel through the intake ports, the fuel minimizes the possibility of carbon build-up. However, on a direct injected engine there is no fuel travelling through the intake valves to clean them. Over time, this leads to intake valves becoming "gunked-up" with solid carbon deposits. This, in turn, affects air flow as the carbon begins restricting the volume of air that can travel through the intake valves. A few symptoms of carbon build-up include:
- Performance decrease
- Rough idle/engine vibration
More carbon may build-up on the cylinder 2 intake valves, for example, as compared to the remaining cylinders. This causes inconsistent airflow to the cylinders and may result in misfires and/or timing differences between cylinders. Additionally, the inconsistent airflow may cause rough idle and vibrations, or stuttering acceleration. Fortunately, intake valves can be cleaned but it is not the easiest job. A common option is walnut blasting the BMW intake valves, which typically costs anywhere from $400-$1,000 (depending on the specific BMW engine and the shop). DIY jobs can be accomplished with minimal cost, but will likely take 3-6 hours for an experienced DIY'er.
Direct Injection on Modified BMW's
Lastly, due to the high pressures of DI, the injectors may struggle to keep up with the fuel flow required for highly modified BMW's. Though, they are still able to flow incredible amounts of fuel so this is likely a non-issue for most BMW owners. However, since we do tailor our content to enthusiasts looking to push their BMW's to the max, we decided to touch on this briefly. Using the N54 as an example, the stock direct injectors max out around 500-600whp depending on the amount of E85 or race gas used.
To our knowledge, there are no aftermarket direct injectors capable of flowing more fuel on the N54. In order to push the N54 further, it actually requires the addition of a second fuel rail utilizing port injection. Again, this is likely a non-issue for 99.9% of BMW owners as the stock injectors support nearly 2x the stock horsepower. However, when the stock direct injection reaches its limit, we must call for port-injection to finish the job.
Final Thoughts on BMW Direct Injection
With BMW's current line-up utilizing direct injection there is no way to avoid the change, unless opting for an older vehicle. However, direct injection has many benefits that, in our opinion, outweigh the downsides and potential additional costs. Direct injection is simply an improved and more efficient design as compared to port fuel injection. Fuel economy and emissions improve by 10-15% all while boosting performance and lowering cylinder temperatures.
What could there possibly be to complain about? At least until an injector or HPFP fails. Worry not though - BMW experimented early with the N54 engine and has since drastically improved the design of their direct injection systems. Direct injection really is a win-win. BMW's direct injection keeps the tree-huggers happy (we're all for being green, too) all while improving performance for us enthusiasts.