Unlocking the nuances of combustion is the key to the next generation of cleaner-burning diesels.
Manufacturers of off-road diesel engines are in a tough spot. First, they're under the gun from the EPA, European Union, and other regulators to cut emissions. They must contend with an increasingly stringent series of standards mandating fewer hydrocarbons, nitrogen oxides (NOx), particulates, and other pollutants from engines used in dozers, excavators, farm tractors, and other nonroad equipment.
Developing new engines that meet the standards is alone a serious technical challenge. At the same time, OEMs are demanding that any new engines be the same size and shape as older versions, and not require additional equipment such as larger radiators or catalytic converters. The powerplant is already shoe-horned into machines like skid-steer loaders and mini excavators, so they don't want to face a costly redesign of their equipment to make the new engines fit.
It's just the opposite for people who buy and use the equipment. If they're forced to upgrade, the new engines had better deliver more power and torque, good fuel economy, along with higher quality, reliability, and durability.
Coming up with products that satisfy everyone has engine manufacturers investing millions in R&D. With ever-tighter standards phasing in every few years, designers are severely pressed to develop, test, and certify an engine and get it to market before the next wave of legislation comes along. But complicating matters, there are several ways to reduce emissions, and future standards the companies will have to meet aren't yet even finalized.
Making diesel engines that burn cleaner and emit fewer pollutants is no easy task because in-cylinder combustion is an incredibly fast and complex process. "In simple terms, we're mixing air and fuel together and just trying to tailor the burning of that fuel in the cleanest possible way," says Allister Dennis, a product manager with Perkins. "Acert controls the way the fuel is burning, the speed at which it burns, and the things like NOx and particulates that form as it burns," he adds.
Several critical elements make up Acert, according to information from Caterpillar. First is the air system, whose goal is to get as much clean air into the cylinder, as uniformly dispersed as possible. In older engines an inconsistent air supply results in incomplete combustion and higher emissions.
The challenge to properly distributing the air charge is there's only a few thousandths of a second to do it. The combustion chamber is also a dynamic environment with vast differences in pressure and heat from one spot to another and a moving piston that constantly changes the chamber geometry. Cross-flow heads, four valves per cylinder, and turbochargers are among the possible ways to improve air control.
The second element is a high-pressure fuel system. Optimizing combustion requires that just the right amount of fuel be delivered at the right time under the right pressure. Providing microbursts of fuel before and after the main injection can better control emissions. Managing these brief events requires highly precise and responsive injectors. An added benefit of this sophisticated rate shaping is better control of noise, vibration, and harshness.
Substantial computing power is needed because the engine is not an isolated component, but a part of a comprehensive system. Letting the engine communicate with other parts of a machine is critical. With suitable electronics the engine can tie into a CAN bus and exchange information with other components. Engine-operating parameters are then fine-tuned to match such things as the demands of the hydraulic system, ambient conditions in the operating environment, and even the actions of the operator. This interactive communication not only reduces emissions, it also improves performance.
But what really ties all the elements together is combustion engineering expertise. The difficult part of Acert, says Dennis, is the vast number of variables and the amount of time spent researching how to tune them to deliver the desired results. For instance, there are countless variables that just concern fuel-injection timing, he says. Critical factors include how early or late fuel is injected, and how many times it is injected throughout a single combustion cycle.
Over the past three years Perkins engineers have been leveraging the expertise of their Caterpillar colleagues to develop the combustion models that deliver lower emissions. "There's a huge amount of CFD work which has taken place around things like piston and bowl design, and understanding how fuel is mixing in the cylinder, which is so critical. And as you get to lower emissions levels, smaller and smaller changes become more and more important," says Dennis. The computer modeling goes hand in hand with a multimillion dollar investment in an array of automated test beds that run around the clock.
Some of the breakthroughs come through work on the desktop, and others are the result of physical testing, he adds. "We've built up a huge quantity of data to understand what changes will lead to the best results," and to develop the skills and combustion expertise necessary to make Acert work.
The effort seems to be paying off. Perkins first success involves its 1100 Series, a 1.1-liter-per-cylinder family of engines that currently meets EPA Tier 2 and European Stage II emissions levels. According to the company, it will be the first time that Acert-type technology has been applied to engines of this size that meet Tier 3/Stage III requirements.
The 1100 Series range will not only meet the new emissions levels but also provide greater power. Currently rated from 65 to 175 hp for the six-cylinder engines, Tier 3 versions will increase power up to 30% and improve torque up to 40% on some models.
The new engine meets tighter emission levels without requiring exhaust aftertreatment. Service periods and maintenance schedules will remain unchanged. The same goes for efficiency, with no change in fuel economy going from Tier 2 to Tier 3. "That's a tremendous achievement in terms of how the technology has matured and how successful our development programs have been," says Dennis.
Another benefit is that the redesign builds upon the proven Tier 2 platform. Introducing radically new technology tends to make people nervous, says Dennis, but that is not the case with the new 1100 Series. "Acert technology changes several things, but it doesn't tear everything up and start over again." For instance, electronic controls that were an option on the Tier 2 products have evolved in making the transition to Tier 3, but the switch is virtually transparent to the OEM and end user.
That's why Acert-type solutions are preferred over alternative technologies, according to Dennis. For instance, cooled exhaust-gas recirculation, a competing technology, is effective at reducing emissions but can introduce a host of service and maintenance issues, increase fuel consumption, and cut reliability.
That's because cooling exhaust gases can significantly increase the heat load to the radiator. This typically means larger radiators that require more space, and bigger fans that draw more power and generate more noise. Recirculating gases which contain sulfur from the fuel also expose the engine to potentially harmful acid compounds.
Acert, on the other hand, offers the customer lower noise levels, low heat rejection, and keeps fuel economy at today's levels, he says. "OEM's will avoid costly reengineering, there will be no need for expensive and complex additional mechanical components and the attendant costs that go with it. Heat rejection will be minimized and package size remains the same as today's engine so there's no adverse impact on end users or on the serviceability and reliability of OEM's finished equipment," says Dennis.
In designing an engine that meets Tier 3 standards, Perkins engineers are also looking ahead to still-preliminary Tier 4 standards that will not phase in until 2008. For instance, cylinder pressure may increase at Tier 4, so current designs ensure the engine has sufficient strength to handle higher loads.
The aim, again, is a smooth transition for the customers between Tier 3 and 4. "The intent is to ensure that the platform, the engine size, the cylinder head, and block structure will carry forward into Tier 4 and Stage IIIB," says Dennis.
Some experts propose that additional systems, such as variable valve actuation and series turbochargers (two standard turbos in series) may be necessary for large off-road engines to meet Tier 4 emissions levels. But Dennis doesn't believe these will be needed on the smaller Perkins engines.
"In Europe we're going to be seeing legislation that forces us to bring particulates down considerably, probably a ten-fold reduction at Stage IIIB." Combining an Acert-based solution with an oxidation catalyst or some other form of aftertreatment is the most-likely solution, he says. "Our strategy is to get the engine emissions as low as possible, so anything we bolt on or change subsequently in Stage IIIB, Tier 4, becomes a smaller change."
Dennis is confident they can meet the next step. "I wouldn't call them major technological hurdles, but the legislation hasn't been approved so it's difficult to comment. The technology is there, it's just applying that to the applications and different uses our engines see," says Dennis. "We think we've got an approach that can get us to Tier 4."
Ausa, based in Manresa, Spain, is a major manufacturer of small dump trucks, off-road forklifts, and other construction equipment. Like other OEMs, they want engines that meet the latest emission standards, fit in their current machines, and offer better performance, quality, and durability.
The 1100 Series engine uses a 1,600-bar (23,500-psi) fuel system and advanced electronic controls to meet Tier 3 emission standards. It offers 30% more horsepower and 40% more torque than Tier 2 versions
Around-the-clock testing and extensive computer modeling have played key roles in Perkins development of the Tier 3, 1100 Series diesel engines.
The U.S. Environmental Protection Agency has set emission standards for diesel engines used in a wide range of nonroad construction, agricultural, and industrial equipment and some marine applications. The goal is to reduce the harmful health effects of ozone and particulate matter (PM).
Standards cover hydrocarbons, nitrogen oxides (NOx), carbon monoxide, and PM, and are to be implemented in a series of tiers. Each tier involves a phase-in, by horsepower rating, over several years.
Tier 1 standards were adopted from 1996 to 2000. More-stringent Tier 2 standards are being phased in from 2001 to 2006, and Tier 3 from 2006 to 2008. This year, the EPA has proposed new Tier 4 standards that would take effect from 2008 to 2014, and would reduce emissions of PM and NOx from today's engines by more than 90%.