Terex Roadbuilding was the first transit-mix truck manufacturer to incorporate the new 2007 EPA emission engine technology into its full line of mixers. Many are now operating in the field. The first Terex Tier III mixer has more than 15,000 miles on it, with zero engine downtime logged.
At last fall's meeting of the Technology and Maintenance Council (TMC), the biggest question was, “How are the 2007 engines performing?”
The answer is, there's good news and there's bad news. The good news is that the new engines seem to be performing well. The bad news is that many of the accessories are not.
This is the response because some of the larger fleets have bought several hundred test EPA 2007 engines for evaluation. Many of these test fleets are TMC members. These managers reported how the units are doing with about 75,000 to 85,000 miles on their engines.Cleaning the air
To understand the problems encountered, it is important to understand what is being done to clean the air. The EPA has targeted two major groups of pollutants for emission reduction. These are particulate matter (PM) and oxides of nitrogen (NOx).
PM consists of partially burned fuel (soot), unburned hydrocarbon fuel (HC), and ash from motor oil. The last round of emissions reductions in the 10/02 engines, concentrated on controlling NOx. The 2007 engines build on that and go after PM.
Normally, PM could be controlled by increasing engine combustion temperatures to the level where all HC material is burned completely, turning it into carbon dioxide (CO2) and water vapor (H2O). But when air, composed primarily of nitrogen and oxygen, is subjected to high heat and pressure, as inside a combustion chamber, the nitrogen and oxygen combine to form NOx, a precursor to smog and acid rain.
The strategies that are designed to control one pollutant actually create more of the other. Cool the flame to limit NOx, and PM increases. Raise combustion temperature to limit PM, and NOx increases.
The EPA mandated that 10/02 engines (the previous regulation change) control NOx by directing some of the exhaust gas, already burned and therefore inert, and mixing it in with the incoming combustion air.
The additional PM amount was limited by increasing the volume of intake air with two-stage, or variable geometry turbo-charging, and managing fuel injection with higher injection pressures and variable timing of multiple injection pulses.
This exhaust gas was cooled using engine coolant and recirculated in measured amounts through the intake manifold. The process, known as exhaust gas recirculation (EGR), limited NOx but added soot to the engine, greatly increasing the heat that the cooling system had to manage. Heat taken into the cooling system is dissipated through the radiator. The heated air then flows under the hood, heating the engine compartment further.
To meet 2007 standards, engine designers increased the amount of exhaust put through the EGR system. This option has raised the under-hood temperature even more. It also may inject more soot into the crankcase oil, which required a new oil formulation (CJ-4) or shorter drain intervals.Diesel particulate filters
To manage the increased PM, manufacturers developed a new device called a diesel particulate filter (DPF). The device uses a porous ceramic filter to trap the PM, letting only gaseous exhaust escape. The solids build up inside the DPF and have to be regenerated periodically.
Regeneration is done automatically when conditions warrant. The engine controller determines how much back pressure exists. A small amount of fuel is injected onto a precious metal catalyst, causing the temperature to rise between 1100° and 1200° F, converting all soot to CO2
The system can be deactivated to prevent regeneration under hazardous conditions, or regeneration can be manually initiated. Some engine makers prevent regeneration when traveling at low speeds or when stopped. Inert ash remains in the DPF, which must be cleaned out every 350,000 to 400,000 miles.
The good news is that the 2007 engines and DPFs are operating as they should. Occasional problems with DPF dosing valves were reported, but they seem to be the result of fuel quality, rather than system function.
At 60 mph, a fully loaded ready-mix truck's engine can generate 1.2 million to 1.5 million BTUs per hour. A BTU is the energy needed to raise one pound of water one degree F. Just over half the energy is lost to the cooling system or out the exhaust. That used to be more than 2/3 just 25 years ago.
The 2007 engines return up to 30% of exhaust heat to the coolant through EGR, forcing the radiator to manage as much as 680,000 BTU per hour. With that as background, here's the bad news.