By Featured Writer Mike McGlothlin
In the modern age of diesel technology, the term direct injection (DI) is something we’re all familiar with, or have at least heard about. This is because the majority of all commercially produced compression ignition power plants at the present time feature direct injection. But what about indirect injection (IDI), the pre-chamber style combustion system from yesteryear? And how much different is it from a direct injection system? Short answer: night-and-day different! The cylinder heads, combustion chamber, injectors, injection pressure(s), injection events, compression ratio, and even the pistons are all different between an indirect injection and a direct injection diesel.
It’s time to examine both technologies from top to bottom, illustrate how each one functions, and then compare them head-to-head using real-world examples. Along the way, you’ll find that even though direct injection dominates the current landscape, indirect injection—despite reaching its zenith some 30 to 40 years ago—still makes sense in many applications throughout the world today. And you’ll also discover that despite all the disparities between DI and IDI, the biggest difference lies not in how fuel is injected, but rather where it is injected. Below, you’ll find the most comprehensive look at indirect injection vs. direct injection compiled to date.
What Is Indirect Injection?
Indirect injection, also known by the acronym IDI, is a fuel injection arrangement where a pre-chamber (or swirl chamber) is combined with the primary, in-cylinder combustion chamber. IDI diesel engines typically feature high compression (better than 20:1), unique-appearing cylinder heads which house the pre-chambers, and are often naturally aspirated as opposed to turbocharged. And despite many IDI engines’ ability to produce peak torque at exceptionally low rpm, optimal horsepower is usually achieved at engine speeds that are noticeably higher than when a direct injection diesel engine produces them. The Detroit Diesel/GM-built 6.5L IDI V-8 is pictured here.
What Is Direct Injection?
Direct injection, also known by the acronym DI, is a fuel injection system that distributes fuel directly in-cylinder and precisely into the combustion chamber present in the piston. There is no mixing of air and fuel in a pre-chamber. In comparison to IDI engines, DI versions commonly utilize lower compression ratios (15:1 to 18:1), traditional-appearing, flat-faced cylinder heads, and are typically turbocharged. And while IDI engines tend to turn out peak torque sooner in the rpm range, pound-for-pound DI power plants produce considerably more torque overall.
Injector Spray Pattern In A DI Engine Is Paramount
In a direct injection system, the preciseness of the injection event—and especially the injector’s spray pattern—means everything. The pattern has to be contained within the piston’s fuel bowl, where the flame is intended to be. Spraying fuel on the outer edge of the piston or (worse) on the cylinder wall will drop combustion temperatures and lead to incomplete combustion. This is why the edges around the top of the piston’s fuel bowl are designed to keep fuel in, so the flame can live there.
How Does IDI Work?
The pre-chamber (again, also referred to as the swirl chamber) is located in an IDI engine’s cylinder head(s). The pre-chamber is linked to the main combustion chamber via a narrow, oftentimes angled passage that is sometimes referred to as the throat. During the compression stroke, turbulence is created in the pre-chamber, which improves the air/fuel mix once fuel is injected on the power stroke. Unlike a direct injection engine, fuel isn’t sprayed directly on top of the piston, but rather into the pre-chamber. Combustion begins in the pre-chamber and then spreads into the cylinder beneath it, acting on the piston.
How Does DI Work?
With this form of injection, air within the cylinder, which has been highly compressed (and in turn, super-heated), allows for autoignition once the fuel is sprayed in-cylinder on the power stroke. In a direct injection system, the injector sprays directly into the cylinder (no pre-chamber), with its spray pattern designed to keep fuel droplets contained within the combustion chamber (i.e. fuel bowl) of the piston. Rather than creating turbulence for optimal air mixing in the IDI system’s pre-chamber, turbulence takes place in the combustion chamber that’s integrated into the piston. A properly designed fuel bowl affords optimal air mixing for meeting both horsepower and emission goals.
What Does An IDI System Look Like?
The pre-chambers are the focal point of an IDI system’s design. They are located above each piston in the cylinder head(s) and accommodate the fuel injectors (and usually glow plugs, too), which spray into the swirl chamber on the engine’s power stroke. The pre-chambers can vary in shape and size, but most are spherical in design. During the compression stroke, the shape of the sphere forces air to swirl around within the chamber, creating turbulence. This improves the air/fuel mix for an optimized combustion event. When fuel is injected into the pre-chamber, it mixes with the turbulent, swirling compressed air, and autoignition takes place.
What Does A DI System Look Like?
In a direct injection arrangement, the fuel injector is located directly above the piston, and its positioning allows for slight protrusion of the nozzle into the cylinder. A properly engineered injector spray pattern ensures that all fuel remains in the piston’s combustion chamber. In DI engines that have them, a glow plug will mount near the injector within the cylinder head, and will also protrude into the cylinder. Thanks to not experiencing the kinds of thermal losses inherent to IDI’s pre-chamber system, glow plugs aren’t as much of a necessity on DI engines for cold start aid.
IDI’s And Inevitable Thermal Loss
There are inevitable downsides to having (essentially) two combustion chambers. As both the pre-chamber and primary combustion chamber strive for some kind of balance between the two, their pressure differences create immense turbulence within the IDI engine. And as you might’ve already deduced, considerable heat loss occurs in this process, which is one of the reasons IDI’s possess such high compression. Beyond that, the pre-chamber hinders the combustion burn, which results in IDI engines typically being between 5 and 10-percent less efficient than their DI counterparts.
Air Cell IDI
One way to reduce the thermal losses encountered with the swirl chamber design is found in the air cell chamber form of indirect injection, also known as the Lanova combustion system. In this arrangement, an air cell within the cylinder head and above the piston is linked to the cylinder via a small throat. The fuel injector is positioned in the throat (as is the glow plug). When the injection even occurs, combustion pressure forces the air to flow out of the air cell with high velocity, where it immediately mixes with fuel leaving the injector and vaporizing very efficiently. Because the combustion event predominantly takes place in the air cell throat and main combustion chamber (and not in the air chamber), less heat loss occurs.
IDI Cylinder Heads
The cylinder heads between an IDI and a DI engine are noticeably different. On an IDI cylinder head, the fire deck side will have an opening in the combustion area. This is the pre-chamber’s passageway into the cylinder and is what links both combustion areas together. However, as the injector is positioned to spray into the pre-chamber with the head, there is no provision for the injector between the valves. The photo shown here is that of an IDI cylinder head from a 6.9L V-8 International Harvester application. The 6.9L IDI was the first diesel option available in Ford’s F-250 and larger pickups starting in 1983.
DI Cylinder Heads
In contrast to an IDI engine’s cylinder head, the DI version is flat-faced on the deck side and, as you can see, there is no provision for a pre-chamber. However, there is a bore for injector nozzle protrusion through the deck surface (the larger hole shown), and in applications equipped with glow plugs, there is an additional port for that, too. Given their dominance over the past three decades, DI heads are what most mechanics would consider being the conventional type of diesel cylinder head.
What Do IDI Pistons Look Like?
As we’ve alluded to thus far, the pistons used in an IDI engine differ tremendously from those employed in DI engines—and now you can see it for yourselves. An IDI piston offers an appearance reminiscent of that of one from a gasoline engine, but the shallow indents (or pockets) present on the piston’s face aren’t for the valves. They’re the primary combustion chamber, and the small indention between them is what links the in-cylinder combustion area to the pre-chamber in the head. Although an IDI engine features high compression, a typical IDI piston from a naturally aspirated application won’t have much more than a standard selection of piston rings (typically a barrel faced top compression ring, taper faced second compression ring and a two-piece oil control ring).
What Do DI Pistons Look Like?
Due to not having any pre-chamber provisions for combustion, direct injection pistons integrate their own combustion chamber into the crown of the piston. The geometry of the common direct injection piston is often referred to as the “Mexican hat” design, but it is arguably better known as the “Hesselman chamber,” named after its developer. At the center of the piston’s combustion area, a conical shape (i.e. the Mexican hat) promotes maximum fuel atomization. Around the top inner diameter of the fuel bowl, there is also what’s known as a lip. Believe it or not, the geometry of the lip’s edge is highly important in determining emissions.
Lower Injection Pressure (IDI)
Thanks to the pre-chamber system of an IDI, they typically feature simple, mechanical fuel injection systems where both injection pressure and injection timing aren’t as critical as they are in DI engines. The latter helps keep overall costs associated with IDI engines low—which explains why IDI was chosen in various OEM vehicle applications. The typical peak injection pressures of an IDI engine with a common distributor style (rotary) injection pump and pintle nozzle injectors vary, but it’s common to see anywhere from 2,100 to 7,250 psi. In comparison, the latest DI diesel in the marketplace—which make use of high-pressure common-rail systems—can exceed 30,000 psi. But even back when IDI was more prevalent, the DI engines of the day were still producing 15,000 to 17,000 psi injection pressures.
Higher Injection Pressures (DI)
First and foremost, direct injection engines operate with higher injection pressure because they have to. Ultra-high pressure yields superior atomization. And with that comes a cleaner burn, and ultimately the ability to curb particulate matter (PM) emissions in-cylinder. Precise injection events and higher pressure requirements add considerable cost to a diesel engine, so it goes without saying that direct injection diesels are far and away more expensive than IDI engines—at least as far as the injection system is concerned.
The Disadvantages Of IDI
With what amounts to two combustion chambers, indirect injection engines have considerably lower thermal efficiency than their direct injection counterparts. The added surface area of having two combustion chambers—along with fighting a block and head(s) that is circulating coolant—inevitably leads to substantial heat loss. After all, the heat lost to the combustion system design could’ve been used to force the piston down… Then comes the issue of high compression, which not only makes starting an IDI engine more difficult than a DI version but brings with it a lack of performance potential. With high compression comes high levels of stress (in particular, cylinder pressure), and an IDI engine can quickly reach its structural limits once modified. A cracked cylinder head pulled from a Detroit Diesel/GM 6.5L IDI V-8 is shown here.
The Advantages Of IDI
An IDI diesel engine’s biggest advantage is its overall cost, which can be kept relatively low thanks to the aforementioned “lower” pressure injection system requirements. Not only does this lend itself to the simple injection pump and injector designs, but the lower pressure means the service life of these vital components can be exceptionally long. In addition to the IDI’s lower pressure requirements to support combustion, its fuel system tolerances are much looser than what you’ll find in a modern-day, direct injection common-rail diesel. This makes IDI engines great candidates for running alternative fuels such as waste vegetable oil and different forms of biodiesel.
The Disadvantages Of DI
Once again, the cost is a factor in the IDI vs. DI debate. This time, it’s a disadvantage for direct injection as higher pressure requirements call for more exotic, more expensive injection pumps, injectors, and even fuel lines (or rails). In the electronically controlled realm of DI engines, the ability to precisely control the injection system comes at a price in the form of additional hardware (ECMs, fuel injection control modules, injector solenoids, etc.) as well as engine software. On top of that, these precise components have zero tolerance for microscopic contaminants and tend to require overhauls sooner than their 100-percent mechanical counterparts.
The Advantages Of DI
Let’s face it, direct-injection took over the world for a reason. At the OEM level, it made it much easier to achieve target power ratings and federal emissions standards. At the customer level, an easier-starting, more fuel-efficient, and more powerful engine makes for an effortless selling point. As for fun factors, with its higher injection pressure capability, better thermal efficiency, and the fact that most DI engines are turbocharged, direct injection can support the significant horsepower and torque gains when compared to IDI. On top of that, DI can do it in a cleaner and quieter manner than an IDI can.
Why IDI Engines Have Higher Compression Ratios
To compensate for the loss in thermal efficiency that takes place due to the pre-chamber system, a high compression ratio helps an IDI engine restore some of its efficiency. High compression also creates increased cylinder pressure, which translates into big torque production at very low engine speed. As an example, the kings of the diesel pickup truck realm in the late 1980s and early 1990s were Ford and Dodge. Ford’s naturally aspirated 7.3L IDI V-8 produced by International Harvester turned out less overall torque than a Dodge ¾-ton or 1-ton equipped with the 6BT 5.9L Cummins, a direct injection inline-six with a turbocharger. However, Ford’s IDI produced its peak torque (338 lb-ft) at just 1,400 rpm. The smaller displacement, lower compression Cummins produced 400 lb-ft of torque at 1,700 rpm.
Why DI Engines Have Lower Compression Ratios
The compression ratios of direct injection engines have, historically, always been lower than the compression ratios found in their IDI counterparts. In recent years, engine manufacturers continue to lower compression ratios for several reasons. First, lower compression leads to lower peak combustion temperatures, which means less nitrogen oxide (NOx) emissions. Second, boost pressure from the turbocharger—a component very few DI engines are without—brings efficiency right back into the equation. And third, a lower compression ratio diesel tends to run smoother throughout the rev range.
IDI Engines: At Home With Natural Aspiration
As previously mentioned, the majority of IDI engines leave (or left) the assembly line void of a turbocharger, but that doesn’t mean they weren’t efficient or useless. Take GM’s 6.2L IDI V-8 offered in Chevrolet and GMC pickups from 1982 to 1992 for example. It was far from a hot rod but delivered exceptional fuel economy. Altitude was one area where you could get into trouble with a naturally aspirated IDI—and indeed it was Ford’s biggest sales pitch for offering a “Turbo IDI” engine in its trucks for the ’93 model year. Off the highway, various other naturally aspirated diesels have made their mark—and not only for their cost-effectiveness, but for their durability—in applications like skid steers, generators, RTVs, and even garden tractors.
Turbocharged IDI Engines
Due to their inherent high compression, which as we’ve learned is hard enough on an engine’s hard parts, turbocharged IDI engines need a few reinforcements to deal with the added stress that comes with forced induction. These reinforcements sometimes include heavier fire-rings on the head gasket(s) and armor wrap to hold up to the higher cylinder pressures and temperatures, stronger connecting rods (with bigger beams or that accept larger wrist pins), thermal coated piston tops to resist cracking when exposed to hotter in-cylinder temperatures, keystone top and sometimes even intermediate piston rings for longer ring life, and Inconel exhaust valves for better corrosion resistance. It’s also worth mentioning that boost pressure is generally kept fairly low on turbocharged IDI diesels (less than 10 psi).
Low-Speed Torque Example: IH’s 6.9L IDI V-8
As proof that an IDI engine can produce torque at very low engine speeds, take the 6.9L International Harvester V-8 Ford offered in its ¾-ton and larger trucks from ’83 to ’87. The engine sported a 21.5:1 compression ratio (’84-’87 model years), utilized the common Ricardo Comet Mark V swirl combustion chamber (the preferred method of IDI), made use of a Stanadyne DB2 mechanical injection pump capable of producing 6,700 psi, and was naturally aspirated. The 420 cubic inches, 90-degree V-8 produced its 318 lb-ft of torque at 1,400 rpm, along with a respectable (for the time) 170 hp. By comparison, today’s direct injection diesel in both Ford’s Super Duty and Ram’s Heavy Duty trucks achieve their peak torque numbers at 1,800 RPM.
Mercedes-Benz OM617: The Fuel-Sipping, WVO-Burning IDI From Yesteryear
It wasn’t going to win any acceleration contests, but the 3.0L OM617 inline-five IDI diesel from Mercedes-Benz enjoyed a reputation for fuel efficiency and longevity—and in many respects still does to this day. Produced from 1974 to 1991, the OM617 featured a cast-iron block and head, 21.0:1 compression, featured a 10-valve cylinder head, and had a chain-driven single overhead cam. The mighty Mercedes built 79 hp and 127 lb-ft of torque when it debuted, but many owners were able to report gleaning 500,000 miles or more from the old-school IDI. The OM617 engine would see a rebirth in popularity in the early-to-mid 2000s when it became a hot buy for running alternative fuels such as waste vegetable oil.
Direct Injection, 30-Plus Years Ago
To be sure, direct-injection existed long before the arrival of the 6BT 5.9L Cummins. But when it debuted in Dodge trucks in the summer of 1988, the 359 cubic inch inline-six not only brought DI to the diesel pickup market, but it was the first turbodiesel in the segment as well. Mirroring everything we’ve mentioned so far, the direct injection 6BT featured higher peak injection pressure of 17,000 psi (vs. Ford’s cross-town 7.3L IDI producing 6,700 psi max), lower compression (17.0:1), produced its peak torque at a slightly higher rpm than its IDI competitor but built significantly more torque overall (400 lb-ft vs. 338 lb-ft).
Direct Injection, Today
Meet the latest 6.7L inline-six Cummins-powered Ram truck. In high output form, it produces 400 hp and an astounding 1,075 lb-ft of torque right out of the box. But that’s just the beginning. A variable geometry Holset turbocharger removes the words “turbo” and “lag” from the diesel vocabulary while producing 33-psi of boost straight off the showroom floor, and the latest Bosch high-pressure common-rail injection system makes the Cummins as quiet and clean as it’s ever been. When you buy an H.O. equipped ’21 or newer Ram 3500, you not only get class-leading torque accompanied by 400 hp, you get a direct injection diesel engine that meets the most stringent PM, NOx, and GHG standards in the world.
Electronically Controlled DI Technology Is Yet To Be Surpassed
Already being more efficient thanks to its lack of a pre-chamber, use of higher injection pressure, and more precise injection events, direct injection engines became even more efficient with the advent of electronic controls. Add to that the ability for a modern-day, fast-energizing, solenoid-equipped fuel injector to react in microseconds and carry out multiple sprays per combustion event and you start to see why electronically controlled direct injection diesel engines are everywhere while indirect injection versions have largely been forgotten.