Direct Injection Served at the Smoke and Mirrors CAFE

Written by Don Stewart | 30 September 2008

In the July 2007 issue of Pony Express , this column looked at carburetor theory and in the February 2008 issue, it offered some guidance for calculating the right size (CFM) carburetor for a 4-barrel upgrade. But we all know that carburetors are old technology and that they were left behind many years ago. We talk about them today because our first generation, second generation, and early third generation Mustangs used them. And because owners of those cars need to be familiar with them.

Fuel injection (FI) replaces the carburetor today. It’s an integral part of modern computerized engine management systems and it makes significant contributions to economy, horsepower, and emissions characteristics. Injection is much more efficient than carburetion because it delivers precise amounts of fuel under various operating conditions. Several FI designs have been known for decades so it’s not new technology. But automobile manufacturers in the U.S. didn’t start using it in high-volume production cars until the early 1980s.

Like most automotive systems, FI evolved over the years. Early designs used mechanical actuation but electronics handle the job now. As manufacturers followed the learning curve for electronic control, increasingly sophisticated computer algorithms led to highly developed iterations of the technology.

In the 80s, the most prevalent design was generically called throttle body injection. Ford called it Central Fuel Injection or CFI but all such systems fell under the technological umbrella called “single point fuel injection” (SPFI). SPFI delivered gasoline to the intake air column in a device called a throttle body. The throttle body replaced the carburetor, it mounted on a typical intake manifold in place of the carb, and it had throttle plates just like a carb. SPFI was used extensively until the mid 1990s.

By the mid 90s, a new generation of FI was seeing widespread use and it continues as the primary design today. It goes by the street name of port injection although its formal name is "multi-point fuel injection" or MPFI. It offers better efficiency, more power, and lower emissions than SPFI. And as the name suggests, the point of injection has moved to the intake port where fuel is introduced outside of the intake valve through an individual injector for each cylinder.

MPFI can be simultaneous (all injectors fire at the same time), batched (small groups of infectors fire at the same time), or sequential. Sequential is the most advanced form because each injector fires separately and in synchronization with the opening of its intake valve during the intake stroke of its respective cylinder. In all three MPFI formats, fuel mixes with the incoming air column as it enters the combustion chamber. The volume of air is controlled by throttle plates, just as it was with SPFI.

Similar to SPFI systems, the MPFI computer calculates how much fuel to inject based on immediate engine needs. But because the MPFI air column contains no fuel until the last moment (dry manifold design), a significant advantage of the system is the virtual elimination of uneven mixture distribution inherent with carbureted engines or those equipped with CFI, both of which are wet manifold designs.

Our Mustangs benefited from each of these fuel delivery advancements as Ford sought to satisfy Corporate Average Fuel Economy (CAFE) requirements, emissions, and—in some models—to enhance performance levels in the second coming of the muscle car wars. But cost considerations at Ford and throughout the industry delayed the wholesale use of systems that were more sophisticated than sequential MPFI, until recently, that is. Now we’re beginning to see direct injection (DI) from some of Ford’s competitors. Although more expensive, DI improves fuel economy and provides more power out of smaller engines. The win win situation was promoted by manufacturers as being green while maintaining or increasing power levels. And then the Energy Independence and Security Act of 2007 was signed into law by President Bush on December 19, 2007. The part of the act that really caught manufacturer’s attention was the section that described how CAFE must be at least 35 mpg by the year 2020.

The president’s signature was hardly dry before the automotive PR guys lined up to aggressively hype DI as a major stepping stone toward the new, greener standard. For once, the manufacturers were becoming greener before the government told them to do it. As noted, DI was already on the street in some cases or in the case of Ford, it was sitting in the wings nearly ready for production. It wouldn’t be the total answer to the new CAFE standard but it would make a significant contribution with fuel savings of up to a 20% in certain engine configurations. And it would give the industry some breathing room while they sorted out additional ways to attain 35 mpg.

So what is DI—or GDI, when it’s applied to gasoline engines? Simply put, it’s a fuel delivery system that sprays fuel directly into the combustion chamber under high pressure, high enough to overcome combustion chamber pressure during the compression stroke of the piston. The pressurized fuel is carried in a common rail (a pressurized fuel storage device) and delivered to each combustion chamber by a dedicated fuel injector that opens for a period of time calculated by—and activated by—the engine management computer. Air is delivered through traditional intake valves but it doesn’t mix with the fuel until it’s inside the combustion chamber. Engine speed is controlled entirely by how much fuel and how much air the computer provides in response to such input signals as the position of the drive-by-wire accelerator pedal, the load on the engine, vehicle speed, and a host of other signals. There are no longer any throttle plates in the system. The computer controls the whole deal.

This all sounds pretty complicated and it is. But greens and gearheads alike should embrace it because the improvement in economy won’t sacrifice performance. Furthermore, potential customers will see that automakers have some ideas left in their bag of tricks, ideas that suggest significantly better economy without the need to flood the market with tiny cars that few people want.

Two great things about GDI are its precision and its flexibility. Examples would include ultra lean burn for light-load running conditions, stoichiometric mode for intermediate loading, and full power mode for rapid acceleration and heavy loading. Stoichiometric refers to the chemical balance that gasoline requires to fully burn under normal conditions. That balance is achieved when one part of gasoline (by weight) is burned with the oxygen provided by 14.7 parts of air. The full power mode requires more gasoline in the mixture, perhaps as rich as one part gasoline to 12.5 parts of air. That leaves ultra lean burn where significantly less gasoline is needed, possibly as lean as one part gasoline to 65 parts of air. There’s little doubt that 65:1 would save a lot of gas but will an engine actually run that lean without destroying itself? The answer is yes it will and it will retain good drivability. The technology behind DI is that sophisticated.

Higher compression ratio (yes, higher), unique piston design (to make ultra lean burn possible), variable valve timing, variable valve lift, variable ignition timing, and injection pulses initiated at various points in the piston’s cycle are some of the features that work together to make GDI successful in each mode.

Having said all of this, the question that immediately comes to mind is: How far back in the wings is Ford’s version of GDI? The answer is: Not far. Inside the Oval, the official publication of Ford Performance Racing had a fairly detailed article on the subject in the 2008 spring issue. The article left little doubt that Ford was ready to put cars on the street with GDI as an option, probably beginning in 2009 with the 2010 Lincoln MKS. The first engines will be 3.5-liter V-6s and beyond the GDI design, the package will include twin turbochargers. During development, Ford called the engine Twin Force which had a nice performance ring. But in an effort to sound more green, the name was changed to EcoBoost where it will remain, if advertising copy is to be believed.

Following introduction at Lincoln, Ford reports that they plan to produce half-a-million units annually over the next five years and spread the wealth across Ford, Mercury, and Lincoln models. Reading between the lines suggests that EcoBoost will include several engine displacements and all of them will follow the formula of twin turbocharged GDI. If you immediately think of turbo lag when turbocharging is mentioned, Ford addressed that issue by using very small, low inertia, units meaning they’ll spool-up rapidly with little or no lag. So drivability should be excellent.

All of this is interesting food for thought but the bottom line for us is spelled M-U-S-T-A-N-G. Will Ford mess with the iconic pony car? Rumors abound, including Mustang EcoBoost options as early as the 2010 model year but Ford hasn’t announced an official timeline, except for the MKS. Another question is: Will the EcoBoost family include a V-8? I can’t say one way or the other but nothing I’ve read indicates that V-8s will be developed. From the beginning, Mustang and V-8 have appeared in the same sentence so we can only hope that the V-8 won’t be scratched. My guess is that the MPFI V-8 will continue for now in the GT with the EcoBoost showing up in non-GT cars as either a small V-6 or possibly an I-4.

With those thoughts in mind, focus on another piece of information for a minute. The 3.5 liter MKS EcoBoost engine and the 4.6 liter Mustang GT mod-motor will live in the same neighborhood. Enthusiasts magazines are reporting that the EcoBoost will develop 340 HP (at unknown rpm) and at least 340 lb.-ft. of torque between 2000 and 5000 rpm. Will the public give up the V-8 GT for a more economical V-6 version with similar power? Only time will tell but high volume cars like the Mustang have significant impact on CAFE so our ponies will need to contribute a lot of units with high mpg ratings.

Things like 6 = 8 and 4 = 6 are already showing up in EcoBoost’s pre-introductory PR. The intent is to start a conditioning process with the consuming public. It’s important to make people comfortable with the idea that the performance of a 6-cylinder EcoBoost engine really can be the equivalent of a traditional V-8 and that a small 4-cylinder engine can perform like an MPFI V-6. With $4.00 gasoline, believers may be easy to come by if they understand what’s going on. But regardless of the conditioning process, I don’t think there’s any question that there will be some interesting changes as we move toward 2020. And at some point, the V-8 will probably be removed from most passenger car option lists.

Ford’s going to do whatever they think is necessary to meet the 35 mpg CAFE but before we get too wrapped up with the number 35, there’s an important point to consider. Specifically, the point would be that CAFE is not based on the EPA estimates displayed on new car window stickers. Current formulas guarantee that a weighted average of sticker estimates will always be lower than CAFE because of some fancy footwork by the government.

The Energy Policy Conservation Act of 1975, enacted in response to the Arab oil embargo, included the original formula for calculating mpg estimates with those same estimates used to develop CAFE. But the formula produced mpg numbers that were much higher than drivers experienced in the real world. After several years of complaining, consumers finally got bureaucrats to address the issue so that window stickers would reflect more realistic EPA estimates.

Rather than change the formula for calculating estimates used to develop CAFE, the solution was to factor the numbers. City mpg estimates were factored downward by 10% to make them that much lower on the window sticker and in similar fashion, highway numbers were factored downward by 22%. The resulting estimates were much more realistic and consumers were much happier. But with no change to the CAFE formula, there was suddenly a significant disparity between EPA estimates and CAFE mpg. In 2008, window sticker estimates were adjusted downward again to compensate for the cumulative impact of faster speeds, more rapid acceleration, air conditioner use, and colder outside temperatures. This served to further increase the disparity because the CAFE formula wasn’t adjusted for those changes either.

The government’s arithmetical gyrations amounted to smoke and mirrors. And the greener 35 mpg standard for the 2020 CAFE uses the same smoke and mirrors. The new standard won’t be as green as it sounds unless the disparity-producing loopholes are plugged. In fact, Dan Edmunds of Edmunds.com calculated that a 35 mpg CAFE would roughly translate to a 26 mpg fleet at the consumer level; i.e., real world experience. With the current fleet averaging an actual mpg of about 21, a 24% increase will be needed to reach 26. On the other hand, if you look at the present CAFE standard of 27.5 mpg you’ll see that it will need to increase by 27% to reach 35. In terms of percent improvement, the two are fairly close, making it reasonable to view them as six of one and a half dozen of the other. Just don’t expect CAFE’s half dozen to equate to 35 mpg in the real world.

Whatever happens, we’ll have to live with it but if the CAFE formula doesn’t change, there’s enough light at the end of the tunnel to suggest that manufacturers can satisfy the 2020 standard without the need to develop a completely new template for making cars. And a fleet that retains some desirable performance cars may be the icing on the cake.