Patent Application
20130219861
This invention relates to reducing emissions from vehicles with internal combustion engines, and more particularly to enhancing operation of exhaust aftertreatment devices by using energy generated from vehicle braking activity.
Today's vehicle emissions standards call for major reductions in the oxides of nitrogen (NOx) and particulate matter (PM) emissions from diesel and other lean burn engines. To help meet these standards, engine manufacturers have developed engine-based strategies (such as exhaust gas recirculation (EGR) and engine timing modifications), and suppliers have developed improved exhaust aftertreatment devices such as the diesel particulate filter (DPF), selective reduction catalyst (SCR) and lean NOx trap (LNT). These devices are also referred to herein as “emissions control devices” or ECDs.
The use of DPFs for PM control and SCRs and LNTs for NOx control, together with in-cylinder control methods, has reduced tailpipe emissions sufficiently to meet current requirements for heavy-duty vehicles. However, a major application difficulty shared by these aftertreatment devices is that they are temperature-sensitive with a finite temperature window for good operation.
A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:
The following description is directed to improving the operation of exhaust aftertreatment devices used in vehicles with internal combustion engines. During braking of the vehicle, braking energy is captured and transformed to heat energy. The heat energy is applied to raise the operating temperature of the aftertreatment device, either by heating the exhaust gas at the input to the device or by heating the device itself.
As indicated in the Background, a DPF provides an effective means for reducing the emission of PM from the tailpipe of a diesel or other lean burn vehicle. The DPF works by trapping the carbonaceous and soluble particulate. When sufficiently high temperature occurs with the DPF, it oxidizes the carbon and hydrocarbons to water and carbon dioxide.
The process of oxidizing the carbon and hydrocarbons trapped on a DPF is called “regeneration”. A DPF that uses “passive” regeneration has catalytic material to reduce the temperature needed for oxidation. An advantage of a DPF with passive regeneration is that it may not require active regeneration.
However, the operating temperature of diesel exhaust gas is typically very low. Frequently in practice, the exhaust temperatures are too low to sustain passive oxidation of the PM.
In real world driving, there are many modes of operation that do not produce sufficient heat to maintain passive regeneration. For the most part, highway operation produces sufficient temperatures to allow the passive control of a filter, however, in low load applications such as urban driving where low speeds, deceleration and idle are dominant driving modes, the exhaust gas temperature can be too low to sustain passive regeneration.
Because a DPF will eventually plug if it is allowed to accumulate PM without regeneration, additional means may be necessary to raise the exhaust gas temperature to achieve regeneration. These “active” regeneration systems provide a means to raise the exhaust gas temperature to heat the trapped PM to a sufficient temperature to achieve regeneration. This is typically done by using various engine management strategies (post fuel injection, intake or exhaust throttling), catalyzed devices (such as diesel oxidation catalysts), fuel combustion or electrical heating.
Unfortunately, active regeneration methods require the addition of energy to the exhaust. This requirement is realized by the vehicle operator as a fuel economy penalty. A fuel economy penalty (FEP) has two parts: increased backpressure (FEPP) and the actual consumption of additional energy (FEPR).
FEP=FEPP+FEPR
The fuel economy penalty due to increased engine backpressure (FEPP) can be simply described as:
FEPp=(ΔP/BMEP)·100%
, where ΔP is the particulate filter pressure drop, and BMEP is the brake mean effective pressure of the engine.
The fuel economy penalty due to energy consumption (FEPR) is a function of the amount of energy required to initiate and sustain regeneration and the efficiency of the selected approach. If the regeneration energy is supplied in the form of an additional quantity of diesel fuel (such as with a fuel burner, in-exhaust injection, or post injection), the fuel economy penalty can be described in terms of the additional fuel energy consumed:
FEPR=DC·(1+(((λ*stoich+1)·Cp·Δt)/LHV)·η,
, where DC is the duration of active regeneration as a percentage of duty cycle (%), Cp is the specific heat of exhaust gas (kJ/kgK), Δt is the required temperature increase of exhaust gas (K), LHV is the lower heating value of fuel, and η is the efficiency of conversion of chemical energy to heat energy at the catalyst.
The two components of fuel economy penalty are related to each other. Specifically, an increase in regeneration frequency decreases backpressure.
NOx aftertreatment devices, such as SCRs and LNTs, are also temperature sensitive. As with DPFs, it is difficult to maintain their optimal temperature window of operation for all driving, particularly urban driving.
Because exhaust aftertreatment devices must operate continually, unless their active temperature window is maintained throughout all operation, efficiency of the device will quickly drop off. If the temperature is too low, and one of the conventional means to increase temperature is used, the additional energy will be realized as a fuel economy penalty. The magnitude of the penalty will be related to the increase in temperature required.
An underlying principle of the methods described herein is to increase the overall energy of the exhaust, particularly during urban driving, by recovering energy lost during braking and returning that energy to exhaust. This approach increases the average energy of the exhaust, and thereby facilitates passive exhaust aftertreatment system operation, particularly during light load driving. Using brake energy (waste energy) to increase the energy of the exhaust reduces the fuel economy penalty associated with regeneration and operating temperature window management by elevating the overall average temperature of operation. Additionally, elevating the exhaust gas temperature will improve the efficiency of catalytic EDCs and may reduce or remove the need for active regeneration. The most appropriate application of this approach is urban driving.
Large diesel fleet vehicles (such as buses and delivery trucks) consume a major portion of their usable energy as braking energy due to stop and go operation in city driving. Each deceleration event typically is also a fuel cut event resulting in exhaust cooling. Thus, these vehicles provide an excellent platform for the methods described herein.
For example, the total energy consumed and total braking energy realized on the New York City Cycle (NYCC) for a 1500 kg vehicle may be modeled by computer. For the NYCC, braking energy totals 33% of the total energy consumed.
A DPF temperature model was developed and integrated into the vehicle model. The DPF model was simplified, but produced reasonable results.
The dashed line in
A generator 51 converts the mechanical energy of braking into electrical energy. Methods and devices similar to those used for regenerative braking in electric and hybrid vehicles can be used to convert the mechanical motion of the wheels during braking into electrical energy. The vehicle's existing alternator could be used for this purpose, perhaps slightly scaled up in output.
The electrical energy is delivered directly to the heater 53. Heater 53 may be implemented with various types of heaters, including ambient (outside the exhaust line) or in-exhaust type heaters. Heater 53 heats the exhaust prior to the input to an emissions control device (EDC) 54. Thus, the heat is applied to the exhaust gas in the exhaust line 55 at the input to the EDC 54. Although not explicitly shown, heater 53 may be implemented so that the heating element surrounds the exhaust line so as to evenly apply heat.
If heat is to be applied in a controlled manner, control unit 52 may be used to regulate and otherwise control the flow of electrical energy to heater 53. Control unit 52 may be implemented with simple electronics, or may be a more sophisticated device. For example, control unit 52 may be processor-based and programmed with various temperature control strategies. Control strategies may include maintaining a desired temperature or temperature range, or providing temperature excursions at predetermined times for precise regeneration events. A temperature measurement device 56 may be used at the input to the EDC, or in the EDC itself, to provide temperature data to the control unit 52.
Because the method transfers energy from regenerative braking to exhaust energy, the system does not require batteries or other electrical energy storage devices. More specifically, no ultra-capacitors or battery packs are required. Also, because the method does not augment driving energy, no special drive motors or controllers are required.
