A new era in high horsepower (HHP) engine technology began on January 1, 2015, the official start date of the U.S. Environmental Protection Agency’s (EPA) Tier 4 Final emissions regulations for diesel engines rated above 750 hp (560 kW). Meeting these very low limits for particulate matter (PM) and oxides of nitrogen (NOx) emissions is the most significant technical challenge that HHP engine manufacturers have faced to date.
With every emissions regulations change, there is a clear target to reduce emissions, but for Cummins, there is always the opportunity to deliver engines with even higher performance and lower operating costs. From the start of our Tier 4 Final development program, Cummins has been committed to delivering engines that surpass the performance levels of the Tier 2 engine population operating in North America today with a lower total cost of ownership. With this goal in mind, our engineering teams went out and captured customer needs for our HHP engine applications and that valuable input went on to inspire our Tier 4 Final technology.
Cummins is very pleased to report that our Tier 4 Final HHP technology has achieved—and surpassed—our original goals for the program, providing fuel savings of up to 4%, extending life-to-overhaul by 10% and—most importantly—enabling engine uptime availability equal to that of any Cummins HHP engine in service today.
Our results were validated not only in the test cell, but also through an extensive series of field tests approaching 60,000 hours, which has enabled Cummins to remain consistent in keeping our promise to deliver more productive engines on schedule for OEM introductions. Based on the impressive results from our field tests, we were able to move the start of our Tier 4 Final engine production ahead of the emissions effect date. This means that Cummins engines such as the QSK19, QSK50, QSK60, and QSK95 will be the first Tier 4 Final HHP engines operating in the industry.
Not only is Cummins able to provide the first Tier 4 Final engines for the industry, Cummins Tier 4 Final approach also means our engines achieve very low emissions levels with no increase in engine displacement needed, no change to engine ratings, and the same, or even improved, power delivery. Each Cummins Tier 4 HHP engine also offers a near identical envelope to that of its Tier 2 predecessor. Upgrades include more efficient wastegate turbocharging to improve transient response and a revised piston bowl design for cleaner combustion, but the base engine is essentially the same.
Our Tier 4 Final program was driven with Cummins core philosophy to utilize new or improved technologies, systems, and components we knew would meet the durability and reliability levels demanded for tough HHP applications. We call this "Innovation You Can Depend On."
To meet these criteria, we took the most severe application in the diesel engine industry—large mining excavators typically operating at 75% load factors for as many as 8000 hours per year—as a starting point. No other machines put more stress or shock load on the engine for more hours. As a result, this extreme duty cycle became our simulation base point. If our Tier 4 Final technology could be counted upon to endure the operating conditions of a mining excavator, we knew it would do well in every other application.
Cummins Tier 4 Final engine systems also provide some other advantages. Service intervals are extended to 500 hours, twice that of the Tier 2 engines. Engine life-to-overhaul is increased by 10% which, for a Tier 4 Final QSK60 rated at 2850 hp (2125 kW), can mean a fuel burn of 1,100,000 gal (4,200,000 L), depending on duty cycle.
As part of the drive to enhance reliability, our design teams analyzed every possible point on the Tier 4 Final engines that could lead to unscheduled downtime and took measures to prevent it. A good example of this is Cummins new NanoNet filter media. This media, which is used in fuel filters, is designed to ensure superior fuel cleanliness and protect the fuel system from dust or dirt ingress. This enhanced protection is achieved with a more uniform pore size compared to traditional media, so that NanoNet captures and retains more harmful particles.
Like their Tier 2 predecessors, the Tier 4 Final engines are designed with rebuild potential in mind, and are easily capable of three overhauls. Another major benefit is that these engines do not require any major intervention at mid-life. It is likely that few in the industry would have expected that this level of incremental durability and performance for HHP engines would have been compatible with the move to meet Tier 4 Final very low emissions.
Achieving very low emissions
For our Tier 4 Final HHP engines, the focus of PM removal was essentially in-cylinder, which is enabled by multiple fuel injection events using higher pressures. For NOx removal, the focus was moved to the exhaust stream using SCR aftertreatment technology. Removing PM in-cylinder and NOx out-of-engine simplifies the emissions reduction strategy and allows the engine’s full potential to be realized.
Also, without the need to remove NOx in-cylinder, the latitude available within the combustion formula to reduce fuel consumption is broadened significantly. It also allows the engine to operate at higher load factors while staying within a more comfortable range of cylinder pressure and piston speed.
The inherent advantages of SCR aftertreatment are well known across the engine industry. SCR is now the key technology employed for all Tier 4 Final engines in the 75 to 750 hp (56 to 560 kW) category, as well as for every on-highway truck engine in North America. Cummins SCR population in North America has already passed the one million systems mark—a testament to the success of this technology. It is clear that no other approach can offer such significant performance and fuel saving benefits while meeting very low emissions.
Only Cummins has been able to replicate these benefits by migrating SCR technology to HHP engines all the way up to 4400 hp (3280 kW). We design, manufacture, and integrate the SCR system ourselves. We have been able to apply the full range of our expertise with catalyst conversion efficiency, diesel exhaust fluid (DEF) spray pattern injection, and integration control logic to develop our next-generation SCR system uniquely suited for HHP engines.
SCR requires the use of DEF to initiate the chemical reaction over the catalyst. DEF is easy to use, easy to store, widely available, and is also an important means for saving fuel. Fuel savings are improved because using DEF enables Tier 4 engines to employ advanced combustion optimization techniques to improve engine efficiency and reduce tailpipe NOx through the SCR aftertreatment. The resulting optimized product consumes less total fuel and DEF, saving customers roughly 3% on total fluid cost.
In this way, Cummins choice of technology shifts a key cost savings benefit to the operator, as this level of fuel savings is significant for large displacement engines.
Cummins Tier 4 Final engines are designed with the ability to do more, such as move more tons per hour or reach peak torque faster. They have been proven to achieve peak performance under the toughest duty cycles and the highest load factors in the industry. We are sure that no other HHP engines have ever been designed with more inherent strength. We think all of this adds up to a very compelling rationale for our Tier 4 Final approach.
Jim Trueblood, Vice-President, Cummins High Horsepower Engineering, wrote this article for SAE Off-Highway Engineering.