Patrick Borgqvist

Gasoline Partially Premixed Combustion in a Light Duty Engine at Low Load and Idle Operating Conditions

Partially premixed combustion (PPC) has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in PPC mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions, even at higher loads. The problem is the ignitability at low load and idle operating conditions. The objective of this study is investigation of the low load limitations with gasoline fuels with octane numbers RON 69 and 87. Measurements with diesel fuel were also taken as reference. The experimental engine is a light duty diesel engine equipped with a fully flexible valve train system. Trapped hot residual gases using negative valve overlap (NVO) is the main parameter of interest to potentially increase the attainable operating region of high octane number gasoline fuels. Much lower soot is emitted with 69 and 87 RON gasoline compared to diesel at engine loads 1 bar IMEPgross to 3 bar IMEPgross but the combustion efficiency is significantly lower with gasoline at low load compared to diesel. Combustion efficiency increases with NVO for both diesel and gasoline. The 69 RON gasoline fuel can be run at idle (1 bar IMEPgross) operating conditions without a significant fraction of trapped hot residual gases. The 87 RON gasoline fuel could be run at 2 bar IMEPgross but with a high setting of NVO. There is a clear decrease of net indicated efficiency with NVO because of the decrease in gas-exchange efficiency. To achieve highest possible efficiency for a given fuel, at low load, as low as possible NVO should be used. (Less)

Investigation and Comparison of Residual Gas Enhanced HCCI using Trapping (NVO HCCI) or Rebreathing of Residual Gases

A comparison between throttled and unthrottled spark ignition combustion with residual enhanced HCCI combustion is made. Early intake valve closing and late intake valve closing valve strategies for unthrottled spark ignition combustion are evaluated and compared. Approximately 3-6 percent relative improvement in net indicated efficiency is seen when comparing unthrottled spark ignition combustion with throttled spark ignition combustion depending on valve strategy and engine speed. The relative improvement in efficiency from spark ignition combustion to HCCI combustion is approximately 20 percent for the conditions presented in this study. The rebreathing strategies have the highest efficiency of the cases in this study.

HCCI Heat Release Data for Combustion Simulation, based on Results from a Turbocharged Multi Cylinder Engine

When simulating homogenous charge compression ignition or HCCI using one-dimensional models it is important to have the right combustion parameters. When operating in HCCI the heat release parameters will have a high influence on the simulation result due to the rapid combustion rate, especially if the engine is turbocharged. In this paper an extensive testing data base is used for showing the combustion data from a turbocharged engine operating in HCCI mode. The experimental data cover a wide range, which span from 1000 rpm to 3000 rpm and engine loads between 100 kPa up to over 600 kPa indicated mean effective pressure in this engine speed range. The combustion data presented are: used combustion timing, combustion duration and heat release rate. The combustion timing follows the load and a trend line is presented that is used for engine simulation. The combustion duration in time is fairly constant at different load and engine speeds for the chosen combustion timings here. The heat release rate is fitted to a Wiebe function where the heat release parameter m is found. It is shown that this parameter m scale to the load and the presented trend line is used for simulating the heat release. When the engine is operated with negative valve overlap the mass flow is reduced through the engine. In an engine simulation the valve timings has to be estimated for different intake temperatures and boost pressure levels. By using the intake temperature at intake valve closing as a prediction tool for the temperature at top dead center, the exhaust valve closing timing can be estimated and will then follow the real test results closely as shown in a GT-Power simulation. The turbocharged test engine is an in-line four cylinder gasoline engine with a total displacement of 2.2 l. The engine is direct injected of spray-guided type. To achieve HCCI combustion the engine is operated with low lift and short duration valve timings where the variable negative valve overlap is used for combustion control. (Less)

The Usefulness of Negative Valve Overlap for Gasoline Partially Premixed Combustion, PPC

Partially premixed combustion has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in PPC mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions, even at higher loads. The problem is the ignitability at low load and idle operating conditions. The objective is to investigate the usefulness of negative valve overlap on a light duty diesel engine running with gasoline partially premixed combustion at low load operating conditions. The idea is to use negative valve overlap to trap hot residual gases to elevate the global in-cylinder temperature to promote auto-ignition of the high octane number fuel. This is of practical interest at low engine speed and load operating conditions because it can be assumed that the available boost is limited. The problem with NVO at low load operating conditions is that the exhaust gas temperature is low. While an increase of NVO potentially increases the in-cylinder temperature at intake valve closing, increasing NVO also increases the EGR fraction which lowers the global in-cylinder temperature. The question is to what extent NVO can be used to extend the low load operating region. Investigations on the effect of the glow plug are also included. The experimental engine is modeled with the engine simulation tool AVL Boost to retrieve information about trapped residual gas fraction and in-cylinder temperature with varying NVO and load at low engine speed and load operating conditions. Measured experimental data is used as input to the engine simulation model at all operating conditions. Measured model inputs include valve lift curves, in-cylinder pressure trace and calculated heat-release profiles. (Less)

The Low Load Limit of Gasoline Partially Premixed Combustion Using Negative Valve Overlap

Partially premixed combustion has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in partially premixed combustion mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions, even at higher loads. The problem is the ignitability at low load and idle operating conditions. The objective is to investigate different multiple-injection strategies in order to further expand the low load limit and reduce the dependency on negative valve overlap in order to increase efficiency. The question is, what is the minimum attainable load for a given setting of negative valve overlap and fuel injection strategy. The experimental engine is a light duty diesel engine equipped with a fully flexible valve train system. The engine is run without boost at engine speed 800 rpm. The fuel is 87 RON gasoline. A turbocharger is typically used to increase the boost pressure, but at low engine speed and load the available boost is expected to be limited. The in-cylinder pressure and temperature around top-dead-center will then be too low to ignite high octane number fuels. A negative valve overlap can be used to extend the low engine speed and load operating region. But one of the problems with negative valve overlap is the decrease in gas-exchange efficiency due to heat-losses from recompression of the residual gases. Also, the potential temperature increase from the trapped hot residual gases is limited at low load due to the low exhaust gas temperature. In order to expand the low load operating region further, more advanced injection strategies are investigated. (Less)

Investigating Mode Switch from SI to HCCI using Early Intake Valve Closing and Negative Valve Overlap

This study investigates mode switching from spark ignited operation with early intake valve closing to residual gas enhanced HCCI using negative valve overlap on a port-fuel injected light-duty diesel engine. A mode switch is demonstrated at 3.5 bar IMEPnet and 1500 rpm. Valve timings and fuel amount have to be selected carefully prior to the mode switch. During mode transition, IMEPnet deviates by up to 0.5 bar from the set point. The time required to return to the set point as well as the transient behavior of the engine load varies depending on which control structure that is used. Both a model-based controller and a PI control approach were implemented and evaluated in experiments. The controllers were active in HCCI mode. The model-based controller achieved a smoother transition and while using it, the transition could be accomplished within three engine cycles. The initial deviation in load is unacceptable but can most likely be improved with a predictive mode transition model compared to empirically selected mode transition parameters. Changing the fuel injection method to direct injection instead of port injection is another possible improvement. (Less)

Transient Control of Combustion Phasing and Lambda in a Six-Cylinder Port-Injected Natural-Gas Engine

Fuel economy and emissions are the two central parameters in heavy duty engines. High exhaust gas recirculation rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark ignition engines. With stoichiometric conditions, a three way catalyst can be used, which keeps the regulated emissions at very low levels. The Lambda window, which results in very low emissions, is very narrow. This issue is more complex with transient operation, resulting in losing brake efficiency and also catalyst converting efficiency. This paper presents different control strategies to maximize the reliability for maintaining efficiency and emissions levels under transient conditions. Different controllers are developed and tested successfully on a heavy duty six-cylinder port injected natural gas engine. Model predictive control was used to control lambda, which was modeled using system identification. Furthermore, a proportional integral regulator combined with a feedforward map for obtaining maximum brake torque timing was applied. The results show that excellent steady-state and transient performance can be achieved.

Comparison of Negative Valve Overlap (NVO) and Rebreathing Valve Strategies on a Gasoline PPC Engine at Low Load and Idle Operating Conditions

Gasoline partially premixed combustion (PPC) has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in PPC mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions. The problem is the ignitability at low load and idle operating conditions. In a previous study it was shown that it is possible to use NVO to improve combustion stability and combustion efficiency at operating conditions where available boosted air is assumed to be limited. NVO has the disadvantage of low net indicated efficiency due to heat losses from recompressions of the hot residual gases. An alternative to NVO is the rebreathing valve strategy where the exhaust valves are reopened during the intake stroke. The net indicated efficiency is expected to be higher with the rebreathing strategy but the question is if similar improvements in combustion stability can be achieved with rebreathing as with NVO. The results show that the rebreathing valve strategy has similar improvements on combustion stability as NVO when the same fuel injection strategy is used. This work also includes results with the NVO valve strategy where a fuel injection is added during the NVO. When a fuel injection is added during the NVO, an additional improvement on combustion stability can be seen which is unmatched by the rebreathing valve strategy. (Less)

Transient Control of Combustion Phasing, Lambda in a 6-Cylinder Port-Injected Natural-Gas Engine

Fuel economy and emissions are the two central parameters in heavy duty engines. High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark ignition engines. With stoichiometric conditions a three way catalyst can be used which keeps the regulated emissions at very low levels. The Lambda window which results in very low emissions is very narrow. This issue is more complex with transient operation resulting in losing brake efficiency and also catalyst converting efficiency. This paper presents different control strategies to maximize the reliability for maintaining efficiency and emissions levels under transient conditions. Different controllers are developed and tested successfully on a heavy duty 6-cylinder port injected natural gas engine. Model Predictive Control (MPC) was used to control lambda which was modeled using System Identification. Furthermore, a Proportional Integral (PI) regulator combined with a feedforward map for obtaining Maximum Brake Torque (MBT) timing was applied. The results show that excellent steady-state and transient performance can be achieved.Copyright