Off-highway vehicles may not move at the same speed as trucks or other large road vehicles, but that in itself presents challenges that on-road vehicles do not face. As emissions regulations for exhaust and yet additional requirements for noise continue to tighten, the challenges have also become more complex. Design of thermal management, HVAC systems, and the control of community noise all demand greater attention to detail than before.
Off-highway powertrains are facing similar challenges to their on-road counterparts in heavy trucks. The adoption of exhaust emissions control technologies such as enhanced exhaust gas recirculation (EGR), selective catalytic reduction (SCR), and the fitting of diesel particulate filters (DPF) has placed greater demands on engine cooling systems and introduced a need for tighter underhood temperature control. Exhaust systems need to be packaged in or near the underhood space because access to sufficient ram air for cooling is just not an option.
An off-highway machine complying with U.S. Tier 4 standards offers an example. Equipped with a DPF, the filter system would need to be regenerated at regular intervals to prevent the filter from clogging. During the regeneration phase, the temperature of the exhaust system climbs rapidly as neat diesel fuel is injected into the exhaust stream to raise the temperature to around 600°C (1112°F) to burn off the soot deposits. The only cooling the exhaust system gets is from the cooling fan, so the entire underhood thermal management becomes impacted by having to package this system locally in or near the engine bay.
Exa’s PowerFLOW computational fluid dynamics (CFD) software can provide an accurate representation of the cooling fan flow, the heat generated by the exhaust, and can give the engineers the necessary information to properly evaluate the thermal efficiency of the entire system. It will show whether it's by convection, conduction, or most likely, radiation. The software can accurately predict the temperatures under the hood and what is the driving mechanism for those temperatures—providing the engineer the information needed to solve the problem.
For instance, if it’s convective, the air would need to be redirected, if it’s radiation a heat shield would be needed, but it is often hard to tell without producing a costly prototype for testing.
Off-highway machinery is expensive to design and manufacture. If the OEM then has to re-position the engine, revise the cooling system, re-route exhaust systems, or find packaging space for the DPF, the production cost could rapidly escalate.
PowerFLOW can simulate all temperature profiles—the surface temperatures on all local components. It can accurately predict all the airflow coming from the cooling system and how it behaves in the underhood area. It can look at long-cycle transients where the DPF regeneration cycles happen, or key-off and soak, factoring in how long it takes to heat up and cool down.
Exa uses a patented Lattice Boltzmann-based technology to accurately predict real-world flow conditions. This can reduce the time taken from CAD to initial results to only seven to 10 business days, even using complex geometrical data, which is retained accurately throughout the process. This could compare with several weeks for traditional CFD technologies, which may not be able use fully detailed geometry for the simulations. Moving forward through iteration stages to the final designs can also be a comparatively rapid process.
Noise has also been part of the regulatory regime. For example, in Europe, noise limits have been part of the Euro standards for commercial vehicles, which have been part of the regulatory framework since the early 1990s, although most of the attention has been focused on toxic exhaust emissions.
For on-road vehicles, the focus has been on drive-by noise levels. Now noise levels from off-highway vehicles and machinery are coming under similar scrutiny. But noise from a static or slow moving vehicle presents a different set of issues from a vehicle passing by at comparatively high speed. Manufacturers are presented with issues that they have not experienced before and require a higher level of expertise in acoustic management than may have been needed before to meet impending and expected legislation.
Globalization in manufacturing may also require a common manufacturing standard, so solutions may be built into products sold in markets where the same standards are not yet required. As an example, the European Commission has proposed noise standards for non-road engines and there is currently no proposed noise regulation from U.S. or Japan that is roughly equivalent.
Managing thermal efficiency and noise control for off-highway machines is a delicate balance. It tends to be more heavily focused first in the areas of thermal management to meet the engine cooling and exhaust packaging targets. After the thermal issues have been evaluated through simulation, the acoustic issues of the system need to be addressed. Often these design constraints conflict with one another, and it becomes a significant engineering challenge to meet the cooling requirements while not exceeding noise targets.
In some machines, the exhaust system is also used to help vent the underhood area. In this case, the exhaust stack consists of two concentric pipes and the high-speed flow of the exhaust gases in the central pipe is used to create a partial vacuum in the outer pipe to help pump excess heat away from the engine compartment. In the process, this produces a whistling noise that may not only be irritating, but may also require engineering work to reduce the generated noise.
Through simulation, it is possible to take the full machine, as designed and as it was intended to be built, and accurately predict to within 1-2 dB the noise of that system. The strength of the process is that not only can it predict the noise, but it can also identify the source(s) of that noise and show the engineering development teams how to reduce it.
In many cases it doesn’t require insulation or added cost, but more effective packaging—where to place the fan, the conditions of the shroud, the fan insertion, other equipment in proximity to the fan that is causing other noise. With good engineering, it is possible to reduce the broadband and tone by 6-8 dB.
It is possible to virtually test the machine, understand the acoustic performance of the design as intended, then provide the cooling system engineers and the NVH engineering groups with information to solve a problem without degrading cooling—always a careful balance. Engineers might want to reduce the noise of the system, but it is necessary to still need to meet the cooling targets.
The same techniques can be applied to HVAC systems. The requirements may be driven less by emissions legislation than by duty-of-care considerations for machine operators. They may spend most of the working day in the cab, possibly in extremes of climate ranging from low to high ambient temperatures. As a result, the HVAC system may require a powerful fan to maintain a comfortable working temperature inside the cab—this can also be a source of constant noise.
There may also be a separate issue regarding where the air intake should be located for the HVAC system, which would effectively control the penetration of dust and exhaust gases into the cab. Again, PowerFLOW can accurately address these issues through simulation, reducing the cost and time needed to ensure the best solutions and thus the best product.
Doug Hatfield, Managing Director, Heavy Vehicles North America, Exa, wrote this article for SAE Off-Highway Engineering.