Tractor Design & Testing 3(2+1)

Lesson 6. Engine design changes for emission reduction

2. Emission Standards:

The different emission norms Bharat (Trem) for stage I, II and III for diesel agricultural tractors in India are given below in Table 6.1.

Table6.1.  Bharat (Trem), Emission standards for diesel agricultural tractors in India

Engine Power	Date	CO	HC	HC+NOx	NOx	PM
kW		g/kWh
All	2005.10	5.5	-	9.5	-	0.80
Bharat (Trem) Stage III A
P < 8	2010.04	5.5	-	8.5	-	0.80
8 ≤ P < 19	2010.04	5.5	-	8.5	-	0.80
19 ≤ P < 37	2010.04	5.5	-	7.5	-	0.60
37 ≤ P < 75	2011.04	5.0	-	4.7	-	0.40
75 ≤ P < 130	2011.04	5.0	-	4.0	-	0.30
130 ≤ P < 560	2011.04	3.5	-	4.0	-	0.20

Emissions are tested over the ISO 8178 C1 (8-mode) cycle. For Bharat (Trem) Stage III A,the useful life periods and deterioration factors are the same as for Bharat (CEV) Stage III, Table 6.2.

Table 6.2: Bharat (CEV) stage III with useful life periods

Power Rating		Useful Life Period
		Hours
< 19 kW		3000
19-37 kW	constant speed	3000
	variable speed	5000
> 37 kW		8000

 

3. Engine design changes for Emission Reduction:

Reduced emissions don’t have to come at the expense of engine performance. Engineers utilize a combination of innovative engine design and new technologies to improve fuel economy and performance while meeting emissions regulations.Available technologies for reducing emissions include:

• Charge air cooling

• In-cylinder solutions

• Exhaust gas recirculation

• Turbocharging

• Fuel injection systems

• Full authority electronic controls

• After-treatment

I. Charge air cooling.

Keeping air intake temperatures as low as possible controls NOx. Air-to-air charge air cooling not only reduces NOx, it also improves engine durability and increases low-speed torque and power density. It is the most efficient method of cooling intake air to help reduce engine emissions while maintaining low-speed torque, transient response time, and peak torque. Charge air cooling enables an engine to meet emissions with better fuel economy and lower installed costs.

II. In-cylinder solutions.

i. Combustion bowl and piston ring design.

Particulate emissions have been reduced by increasing injection pressure and improving the shape of the combustion bowl at the top of the piston. A reduced lip radius on the re-entrant bowl piston increases turbulence and air fuel mixing, helping burn all available fuel during combustion. The addition of valve guide seals limits particulates by reducing oil consumption. Directed top-liner cooling reduces oil consumption and enhances combustion efficiency by reducing wear in the top ring turnaround area and improve piston ring performance.

ii. Premixed compression ignition (PCI).

In traditional diesel combustion, the burning occurs in the rich regions of the spray resulting in high temperatures and high NOx. With premixed compression ignition (PCI), multiple fuel injection strategies are used to lower temperatures. This technique reduces NOx without using exhaust gas recirculation.

iii. Exhaust gas recirculation (EGR).

The lower an engine’s peak combustion temperature, lesser the amount of NOx created. EGR is an effective method of reducing peak combustion temperature and reducing NOx. The concept is simple, during certain conditions of engine operation, the EGR valve opens and measured amounts of exhaust gas are routed back into the intake manifold and mixed with the incoming fresh air. Since this process removes some oxygen from the air, the exhaust temperatures in the combustion process are lowered and the levels of NOx are reduced. Cooled EGR, as used in John Deere Power Tech Plus™ engines, increases the effectiveness of NOx reduction, while enhancing engine efficiency and power density (similar to charge air cooling).

III. Turbocharging.

i. Standard or wastegated turbocharger.

A turbocharger is an engine supercharger driven by exhaust gas turbine. Exhaust gases from the engine enter the turbine housing radially and drive the turbine wheel, which drives the compressor wheel. Both being mounted on the same shaft. Turbocharging increases the density of the air delivered to the cylinder thus making its volumetric efficiency 100% well above that available in natural aspiration. It allows the engine to burn more fuel and in turn develop more power.

Depending on the power rating, standard or wastegated turbochargers are precisely matched to the power level and application. Transient smoke is controlled by using higher-boost turbochargers, including using wastegated turbos that increase low-speed torque and prevent over-boosting at high speed.

ii. Variable geometry turbocharger (VGT).

Variable geometry turbocharger, which helps drive exhaust gas recirculation. The VGT tailors the amount of recirculated exhaust gas that mixes with the fresh air. This is accomplished through the engine control unit, which changes the pitch of the VGT vanes in order to maximize power and efficiency. The amount of cooled EGR required is determined by load and engine speed. When the exhaust flow is low, the vanes are partially closed. This increases the pressure against the turbine blades, making the turbine spin faster and generating more boost.

The VGT minimizes the impact on engine envelope size and provides excellent performance across the entire operating range of the engine, including transient response and fuel economy. It is also a highly effective approach to meeting Stage III A regulations and allows a wider power range using common engine performance hardware — reducing the number of engine configurations.

IV. Fuel injection systems.

i. High-pressure common rail fuel injection system.

For middle range engines, the high-pressure common rail fuel injection system provides constant control over fuel injection variables such as pressure, timing, duration, and multiple injections. It delivers higher injection pressures, up to 1600 bar, for more efficient combustion.

 ii. Electronic unit injector (EUI).

For the larger size engines, the EUI fuel injection system is used to increase fuel pressure for more efficient combustion. This helps reduce NOx and PM.

iii. Mechanical fuel system.

To meet many emission regulations for smaller engines can be done by making improvements to the mechanical fuel system. Newer mechanical fuel systems are able to generate higher injection pressures for more efficient combustion.

V. Full authority electronic controls.

Another key component in emissions reduction is the engine control unit (ECU). It uses sensors and models to control fuel quantity, injection timing, air-to-fuel ratio, multiple fuel injections, amount of cooled EGR, and a host of other control parameters to deliver peak engine performance and fuel economy. Integrating electronic controls between the engine and the entire vehicle also reduces emissions and improves performance.

VI. After Treatment.

i. Selective catalytic reduction (SCR).

SCR is an aftertreatment option that requires a urea-based additive to reduce NOx emissions. When ammonia in urea is mixed with engine exhaust in a catalytic converter, a chemical reaction takes place and the NOx in the exhaust is converted to oxygen, nitrogen, and water. This method adds costs because of the extra tank, pump, associated components, and the SCR additive – but it provides better fuel efficiency than other NOx-reducing methods.

ii. Lean-NOx catalyst.

There are two types of Lean-NOx catalysts with different methods of regenerating. The DeNOx catalyst is a precious metal-based system that reduces hydrocarbons in an oxygen-rich exhaust stream. Without using electronic controls, the catalyst’s efficiency for reducing NOx is less than 10 percent. When electronic controls are used, the efficiency is still less than 30 percent – with a substantial fuel economy penalty. A more effective Lean-NOx catalyst is the NOx adsorber (NAC), or sometimes called the Lean-NOx trap (LNT). It must be regenerated in an oxygen-deficient environment, requiring more sophisticated controls. When the unit is clean, it can reduce NOx by 90 percent. However, it is very sensitive to fuel sulfur levels and can lose efficiency quickly (to near zero) when exposed to high-sulfur fuels. When that happens, sulfur has to be removed from the catalyst. This process of sulfur removal is called desulfation. It requires very sophisticated controls and exposes the engine to high thermal stress while running at a significant fuel economy penalty. By using electronic engine controls with this type of catalyst, efficiency can be maintained greater than 60 percent over the life of the engine.

iii. Diesel oxidation catalyst (DOC).

The diesel oxidation catalyst (DOC) doesn’t reduce NOx, but it is effective at reducing carbon monoxide, hydrocarbon, and some particulate matter. The flow-through oxidation catalyst oxidizes both gaseous (volatile) hydrocarbons and the semi-volatile portion of PM known as the volatile organic fraction (VOF). At higher exhaust temperatures, DOCs can also oxidize sulfur in the exhaust to form sulphate precipitated material. Catalyst manufacturers have been able to achieve the needed VOF reduction while minimizing sulfate formation. DOCs operate at peak efficiency when the sulfur concentrations in the fuel are 0.05 percent or lower. DOCs typically reduce emissions of particulate matter by 20 percent. DOCs also reduce emissions of hydrocarbons by 50 percent and carbon monoxide by 40 percent.

iv. Active diesel particulate filter (DPF).

DPFs (or particulate traps) will likely to be one of the options used to meet particulate matter reductions in Stage III B and Final Tier 4/Stage IV. Manufacturers are working hard to reduce the cost and optimize the size of these aftertreatment devices. The DPF traps and holds particulates in the exhaust. Exhaust gas flows through channels with porous walls that allow exhaust to escape, but traps soot and particulates. Then, with the help of a catalyst, the DPF regenerates by burning the collected soot.

Because high exhaust temperatures are required for this regeneration to take place, the challenge is to design DPFs that provide consistent regeneration at all levels of engine operation. “Active” diesel particulate filters solve this problem, by raising the exhaust temperature based on particulate filter backpressure. Manufacturer’s working to develop active DPF systems that would regenerate in low-load and normal operating conditions.

Metals and ash found in lubricating oil can become trapped in the DPF as well. Since ash and metals cannot be burned off during soot regeneration, they are left in the filter. The buildup can eventually clog the filter and may require maintenance and cleaning. The use of lubricating oil with low ash can help alleviate this issue.