First Look: Ford's All-New 6.7-Liter V-8 Power Stroke Diesel Engine

Photos Courtesy of Ford Motor Company

Ford is rewriting the rules of the company and industry with the introduction of its all-new advanced 6.7-liter Power Stroke V-8 diesel engine for the 2011 F-Series Super Duty pickups.

“The 6.7-liter diesel puts the motor back in Ford Motor Company,” said Adam Gryglak, the engine’s lead engineer. “This is the most advanced Power Stroke yet with the cleanest emissions,” lower noise, vibration and harshness “and substantially improved power and fuel economy.”

Codenamed “Scorpion,” the 6.7-liter PSD is Ford’s first-ever designed-in-house pickup truck diesel engine since the first oil burner (International’s 6.9-liter V-8) was offered under the hood of a Ford pickup in 1982.

“This was a global effort within Ford to create this engine,” Gryglak said. “We applied many of the lessons learned from Ford’s European operations.”

Ford’s road to the Scorpion has been long and winding. The 6.7-liter PSD is the third all-new heavy duty diesel engine in 7 years and the fourth since production of the 7.3-liter Power Stroke ended in 2002.

Its creation can be traced back to the 6.0-liter Power Stroke that Ford introduced for its 2003 Super Duty pickups. Ford and Navistar (International’s parent company) went to battle in court over warranty problems and cost issues related to that engine, which ultimately led to both companies ending their 30-year diesel manufacturing relationship in January. Ford kept the rights to the well-known Power Stroke name that has been associated with Ford diesels since 1994.

No more Navistar. The 6.7-liter Power Stroke V-8 is Ford's first diesel motor for its F-Series pickups to be developed entirely in-house after three decades partnering with the contract engine manufacturer. It will be built at Ford's plant in Mexico.

Ford introduced the 6.4-liter Power Stroke for its 2008 Super Duty pickups in order to meet tough emissions laws that required an immediate 90-percent reduction in soot, a natural byproduct of diesel combustion; General Motors and Chrysler did the same.

Clean-air regulations will be ratcheted up again on January 1, 2010. Diesel-powered pickups built after that date will have to have engines that reduce nitrogen oxide emissions by 90 percent from today’s standards and by 96 percent from 1994 standards.

NOx is a major air pollutant that contributes to smog, asthma, and respiratory and heart diseases. It's caused by diesel’s high combustion temperatures, which results from the high frictional heat created by compressing air in the cylinders to the point where it can ignite diesel fuel without using a spark.

But where GM and its engine development partner Isuzu have been able to reuse the basic architecture of the 6.6-liter LML Duramax V-8 diesel engine introduced in 2001, and Dodge is carrying over the 6.7-liter Cummins I6 that’s been in service since 2007, the Ford 6.7-liter V-8 is an almost total clean sheet redesign that uses many unconventional solutions to meet the needs of its drivers and the EPA.

Reverse Flow Design

The Scorpion’s architecture shares several key traits with GM's indefinitely postponed 4.5-liter Duramax V-8 diesel engine. Most notably, the intake and exhaust flow through the cylinder heads is reversed when compared to a conventional diesel engine, with the exhaust exiting directly into the engine’s turbo that sits in the engine's valley, mounted between V-style cylinder banks.

“When we looked at the at the design imperatives of the program we were looking to ensure that we delivered improved performance, fuel economy, NVH and heat management with the aftertreatment system,” Gryglak said. “The reverse flow and inboard exhaust configuration helps us achieve all of those objectives. It’s a smarter design.”

New aluminum cylinder heads replace cast iron to save weight. They feature dual water jackets for cooling and a six-bolt attachment pattern for extra strength. Note the asymmetrical sizes and layouts of the intake (larger) and exhaust (smaller) ports.

Why? The arrangement dramatically shortens the distance between the exhaust and turbo, improving turbo response while protecting nearby powertrain components, like the fuel pump and alternator, from excessive heat. Higher turbo outlet temps also provide extra heat to downstream emissions devices to improve pollution-scrubbing performance sooner while emission catalysts (used to break down harmful pollutants) are warming up.

“Total exhaust volume and surface area of this configuration is about half that of the previous engine,” Gryglak said. “At the same time, we’ve been able to significantly improve the throttle feel of the truck.”

Single Sequential Turbo

Ford also reengineered the variable geometry turbo from the ground up with help from Honeywell’s Garrett Turbo Division (the same turbo supplier for the Duramax). As modern diesel mills like the 6.7-liter have become more powerful from generation to generation, emissions limits have gotten much tighter, and it’s become progressively difficult to match the turbo’s dual jobs of exhaust gas recirculation (EGR) with acceptable levels of responsiveness.

Instead of using two separate sequential turbochargers like the current 6.4-liter PSD, the new Single Sequential Turbocharger places two compressors back-to-back (right side of picture in blue, representing cool intake fed air) to create a twin turbo setup in a single package. The VGT turbo still uses a variable vane setup around the turbine (left side of picture in red, representing hot exhaust) that continually change the velocity of the exhaust gas and wheel speed.

EGR recirculates some of the engine's exhaust back into the engine at a lower temperature. The cooled gases have a higher heat capacity and contain less oxygen than air, lowering combustion temperatures and reducing the formation of NOx.

Multi-turbo systems, like the previous Borg Warner two-turbo sequential setup that the 6.4-liter PSD used, can overcome these issues but they also face problems in packaging and heat that wouldn’t work with the Scorpion’s new architecture. 

“The turbo we’re using for the 6.7-liter engine is called a ‘Single Sequential Turbo,’” Gryglak said. “It’s a triple-wheel design with a single turbine and two compressors placed back-to-back. What we’ve done is take a twin-turbo configuration and package it into a single unit that gives us seamless transitions [as its adds boost throughout the RPM range and power band]. It also allows us to drive EGR at very low airflow rates to meet the new NOx regulations but also allows us to build the boost we need to overcome the pressure from the EGR.”

The Siamese compressor wheels are near-mirror images of each other. They have the same diameter and are optimized to reduce pressure differences that could cause noise or airflow issues. By packaging them as two smaller turbos, mass is also kept closer to the shaft which helps with balance and durability. The turbo is quicker to boost and better able to deliver horsepower and torque, especially at the low end where it’s needed most the help get heavy loads moving quickly.

External picture of the Power Stroke's new SST turbocharger.

The SST turbo continues to use variable vanes that surround the turbine wheel to dynamically adjust turbo speed using exhaust gases. During engine operation at low speeds and load, the vanes are closed to accelerate exhaust gases across the turbine wheel to help increase turbo wheel speed quickly. At high speeds the vanes are opened to help prevent the turbo overspeed. It's also been enhanced to introduce exhaust braking into the platform.

“You’ll get the feeling [when you’re driving in] tow/haul mode but it won’t explicitly be called exhaust braking. It’s built into the system,” Gryglak said.

The turbo also uses a brand new ball-bearing cartridge that surrounds the turbo shaft to help provide a double-digit increase in spool-up times.

Though some turbos are mounted to the engine off the turbine side – a solution that can cause balance issues requiring extra structural reinforcement and noises like whistles and whines – Ford balances the SST by mounting it at the turbo’s center using a 4-bolt pedestal housing. The compressor stage is bracketed back to the pedestal so it has an extremely stiff structure in front. Separate oil and water feeds flow through the pedestal to lubricate and cool the turbocharger and eliminate as many external connections as possible. The front of the pedestal houses the turbo’s oil filter.

The 6.7-liter SST provides up to around 30 psi boost compared to approximately 40 psi from the 6.4-liter dual-sequential unit and operates at up to 130,000 rpms.

New 'single plane' rocker arms are individually attached to each of the 4 valves per cylinder (2 intake, 2 exhaust) instead of using a bridge to open or close the valves in tandem.

Two Cooling Systems

The Scorpion has two separate cooling systems, each with its own radiator and water pump.

The primary cooling loop cools the engine and operates at about 194 degrees. A secondary 122-degree loop controls the temperatures of the EGR, fuel and transmission coolers and a brand new air-to-water charge air cooler that replaces the previous air-to-air intercooler. The CAC sits between the turbo compressor outlet and the intake manifold to cool the air heated by passing through the twin turbos.

“Overall, the cooling pack is significant smaller” than the 6.4, said Ed Waszczenko, lead durability engineer for the Scorpion. Waszczenko was responsible for testing every component of the Power Stroke by pushing them to their breaking point to ensure durability. “The engine is more efficient. We need less cooling. A lot of the heat management of the engine has helped us reduce the cooling requirements.”

EGR and SCR

All of the engine’s EGR comes off only the right cylinder bank. Ford did extensive research that showed EGR could be pulled from a single bank instead of both sides of the engine to reduce the plumbing required. It also eliminates airflow balance issues that can occur when pulling EGR gases from two cylinder banks.

The 6.7-liter EGR system uses two EGR coolers, like the 6.4-liter setup, but it introduces a “hot-side valve” at the front of the first cooler that controls the volume of air let into the system instead of using a conventional “cool-side valve” behind the second cooler.

EGR is only pulled from a single engine. It's the first step in scrubbing NOx emissions to meet 2010 EPA rules. The second step is the application of Diesel Exhaust Fluid (aka urea) in the exhaust stream, which will require periodic refills.

Ford says the move to a hot-side valve was a lesson learned from its diesel experience in Europe, where other hot-side systems have been engineered to avoid the valve getting jammed from particulates.

“Cold-side applications have extensive warranty issues for valves stuck open due to soot deposits,” Waszczenko said. “The challenge for us is to get [the hot-side valve] enough cooling so the valve is durable for 250,000 miles. There’s an iron valve and aluminum valve, both water-cooled. They expand differently based on their construction. The F-550 and up will use the iron valve. Lower engines will use the aluminum valve.”

Three other key design EGR features include:

• A floating core design, instead of clamshell, that allows the EGR coolers to independently move within their housings as they thermally expand and shrink.
   
• An EGR bypass valve directs exhaust gases straight into the induction system when the engine is cold during startup to get EGR working as soon as possible to lower NOX levels.
   
• And a repurposed throttle body that’s used like an EGR valve to drive the correct amount of EGR that the system requires at the right pressure.

Waszczenko says all the changes have made the EGR system more durable. “We have not failed a single EGR cooler during testing,” he said.

But EGR alone isn’t enough to reduce NOx to meet clean air standards. The 6.7-liter PSD also introduces selective catalytic reduction to Ford’s pickups.

SCR uses diesel exhaust fluid, a urea-based solution (32.5 percent industrial urea and 67.5 percent deionized water), that’s injected as a fine mist into the engine’s hot exhaust gases. The heat turns the urea into ammonia that – when combined with a special catalytic converter – breaks the NOx down into nitrogen gas and water vapor. It's similar to the approach used by Chrysler for its 2010 Dodge Ram 3500/4500/5500 cab chassis trucks.

Diesel exhaust fluid refill intervals will vary depending on duty cycle. Some customers will only have to refill during routing maintenance, such as when the oil is changed, while others will have to top off the tank sooner.

“We think we’ve found the right balance between what the engine EGR can deliver and what the DEF system can do,” said Chris Oberski, emissions engineer for the 6.7-liter Power Stroke diesel engine.

Airflow

Airflow cutaway showing the throttle body (which is used primarily to assist with the exhaust aftertreatment) and intake manifold that's made from composite material instead of aluminum.

Air comes into the engine through the airbox and is directed passively through upper and lower chambers into the compressor portion of the turbo. At the same time, hot combusted gases from the engine are cooled using EGR. Both the cold compressed air and hot EGR are mixed together in the EGR throttle body and then directed into special chambers on either side of the intake manifold. The manifold uses tuned resonators to cancel out any pressure waves that are created within the intake system that could be heard as transient airflow noise, such as a drone or moaning sound.

The intake manifold is made from a composite material. Gryglak said that using aluminum would have created a heat sink that would have robbed power from the motor.

Special flow-mapped rocker covers that seal the valvetrain and provide structural rigidity also have feed chambers that direct air to the intake valves and then into the cylinders for combustion.

Combustion Components and Capability

Every moving part of the Scorpion’s cam-in-block design is new, from the crank to the pushrod valvetrain.

“It takes a fundamentally different combustion system to achieve lower feedback noise levels from the engine and maintain, or in our case, reduce emissions,” Gryglak said. “We’re also promising significantly improved fuel economy.”

The forged connecting rods are attached to the steel crankshaft using a 45-degree slant opening in the rod, instead of 90 degrees, for easier assembly. New cast pistons are gallery-cooled to control temperatures, with one entry and one exit for oil to flow through in the bottom of the piston. When the piston descends during the combustion cycle, a cooling jet of oil is fired into the entry. Special oil drillings add extra lubrication to the joint where the rod and piston are joined.

The pistons (top) have a unique bowl shape design that helps reduce noise and emissions during combustion. The connecting rods (middle) are attached to the crank using a 45-degree opening. 3rd generation piezo electric fuel injectors (bottom) have 8 holes and can squirt fuel up to 5 times per stroke.

Ford officials said they spent close to two years to find the optimal balance between in-cylinder noise and emissions during combustion.

The top of the cast-aluminum piston is very unique. Its bowl-like shape plays a key role by helping control the chaotic swirl of the fuel-air charge during combustion. Ford and piston supplier Federal Mogul cycled through dozens of CAD iterations and twelve physical designs to create just the right concavity for the piston top, according to Oberski.

The Bosch common-rail fuel system operates at up to 30,000 psi to send fuel to the engine’s 8 piezo injectors -- one per cylinder. Each injector nozzle has 8 holes and can deliver up to 5 injections per combustion cycle. Two pilot injections control noise levels, and a main injection is used for the power-generating combustion event. Two variable post injections are used to produce extra torque and for heating up emissions catalysts that scrub NOx and eliminate soot downstream from the engine. While cruising, only 3 or 4 injections will be needed.

“We’re using the third-generation of this technology,” Gryglak said. “It’s key to meeting our fuel economy and emissions objectives. It allows us to better atomize the fuel to mix it with the incoming air for improved combustion efficiency.”

The compression ratio drops a bit, to 16.2 from 16.7.

Special acoustic covers sit over the injectors on each cylinder bank to quiet impulsive noises from the fuel system as well as to lower overall NVH from the engine.

The high pressure common rail fuel system operates at up to 30,000 psi

The asymmetrical layout of the intake and exhaust valves is the last approach needed to manage noise and emissions. It’s a four-valve setup, like the 6.4-liter engine, where the two intake ports are larger than the two exhaust ports.

The overhead valvetrain also uses a new rocker-arm configuration to open and close the two intake and two exhaust valves. It’s designed to improve the engine’s wear characteristics. Four pushrods per cylinder (two per hydraulic lifter) each have a single-plane rocker arm that’s individually connected to one of the four valves instead of using a conventional bridged valvetrain setup that runs multiple valves in tandem. It’s the first time Ford has created such a solution.

Instant start glow-plugs provide gas-like starts at very cold temperatures.

New aluminum cylinder heads reduce engine weight and incorporate dual upper and lower water jackets to provide cooling to the valvetrain.

The engine crankcase has shifted from conventional gray iron construction to state-of-the-art compacted graphite iron. It saves weight while boosting strength.

“What you’ll notice in the shape of the [engine] block is the structure that’s been added,” Gryglak said. “It has very thin wall sections. We’ve added structure to where it’s needed, but still saved overall weight. From a weight to displacement ratio, this will be the lightest block in the segment. The overall engine platform is about 160 pounds lighter than 6.4-liter diesel.”

The engine uses a two-piece oil pan to help store its 13 quart capacity. The bottom piece is composite to help quiet the engine.

The engine block is the first use of compacted graphite iron instead of conventional gray iron that saves weight while gaining strength. Note the prominent ribbed structures where additional reinforcement has been added.

After all the radical changes to the engine, the overall packaging is the same as the last engine. The 2011 Super Duty won’t require any frame modifications.

Ford hasn’t provided power figures yet but promises they’ll be significantly greater than the outgoing motor. Expect peak horsepower at 2,800 rpm and peak torque at 1,600 rpm, Ford officials say.

We witnessed the engines being tested in an acoustic dyno chamber and can report back that noise levels are noticeably lower on the 6.7-liter engine than the 6.4-liter PSD.

Transmission

Ford is quiet for now about the Scorpion’s transmission, though we expect the Super Duty's new gearbox will be the new 6R140 heavy-duty six-speed automatic with power takeoff capability.

Ford officially says the standard manual transmission is gone as of the 2011 model year. The take rate was too low to justify continuing production of the ZF-source 6-speed handshaker. The rear cover of the new engine follows an SAE 12 bolt standard that allows Ford to mate almost any HD automatic transmission to the back of the mill.

Biodiesel Compatibility

The Scorpion diesel is certified for compatibility with B20 biodiesel (80 percent standard diesel, 20 percent biodiesel blend), like the 2010/2011 Cummins and Duramax diesels.

“We’ve protected the customer if they want to use B20,” said Gryglak “Biodiesel blends can sometimes vary in quality, so on our low-pressure feed lines into the high-pressure fuel pump there’s a pressure switch. If there’s a bad batch of fuel, sensors will immediately notify the customer they have an issue [through] the trip computer.”

Ford recommends that customers running biodiesel in the 6.7-liter engine not to let the fuel sit for longer than a month. Otherwise, things can start to grow.

Testing the Engine

Scorpion engine in a dyno test cell.

Ford says it has put the 6.7-liter engine through a vigorous testing scheme to identify any potential weaknesses or quality concerns before the first units wind up in the hands of customers.

A 250,000 mile durability test looks for structural fatigue points. It simulates the driving habits of 95th percentile Super Duty customers who pull the heaviest loads. The engines are placed in a dyno cell where they spend nearly 6 hours running continuously at peak torque and then 3.5 hours at peak grade power to prove out connecting rod and rotating mass strength. The cycle repeats for 1,200 hours, or 50 days of running.

A thermal fatigue test is used to prove out the engine assembly process by stressing the head gasket, joints, radiator connections and other seals. For this test cycle, the engine is idled and then throttled up to peak power for about 14 minutes. That’s enough time for the engine to get red hot. It’s then shut off and 16-degrees below zero coolant is pumped in and allowed to soak for a few moments before the engine is fired back up to rated power. The shock loop is 150 hours long, or 75 cycles.

A structural test is run to make sure the engine’s build tolerances and parts acceptance criteria are set properly, so there are no surprises during production. Engines are built using actual parts with purposely incorporated defects that simulate the worst quality issues Ford has seen in production, such as low head bolt torque or inclusions in the piston casting bowl. It’s a new test that the Scorpion team determined was necessary if the engine was going to be built in-house.

Finally, there’s a real-world wear test that, like the durability test, simulates the operating conditions and applications of Ford’s 95th percentile customer. Fully assembled Super Duty engineering test mules are run over 250,000 miles of the most grueling roads in the U.S. with the toughest grades in temperatures that vary from -40 to 130 degrees. It’s the equivalent of 10 years of services in six months. Two-hundred test points are measured at peak rated power and torque over every speed range to make sure the engine’s entire real world operating spectrum is tested.

At the end of each of these tests, the engines are broken down and the team examines every component, looking for problem areas. When problems are found, they’re fixed and then the test cycles start over again.

Putting Their Money Where Their Mouths Are

Ford won’t say how much money it invested in the 6.7-liter Power Stroke diesel, but we think it may be the most expensive single engine program in the company’s history. Perhaps we’ll see more Scorpion derivatives to spread the investment among a larger group of vehicles.

Ford also isn’t talking about the prices when the engines go on sale in the 2011 Super Duty next year. We think it will run more than the current engine. But Ford does repeat over and over like a mantra: the 6.7-liter PSD will be the most powerful, most fuel-efficient and the most-refined Power Stroke diesel engine yet. And the team that created it says it will be around a long time.