1. What is a low temperature combustion engine?
The goal with a low temperature combustion (LTC) engine is to achieve high levels of fuel efficiency without producing harmful emissions. Oxides of nitrogen (NOx) and particulate matter (PM, also known as soot) are the two main regulated pollutants in diesel combustion. Figure 1 shows the relationship between flame temperature and pollutant formation. NOx formation decreases as flame temperature decreases, and PM is minimized with lean combustion. LTC engines burn cool enough and lean enough (low equivalence ratio) to stay out of the high soot and NOx formation zones, yet they are still able to take advantage of the high thermal efficiency of typical compression ignition engines: they have high compression ratios and ideally operate without a throttle. Numerous LTC engine concepts are being researched at universities, laboratories, and agencies across the world. Below is a list of a few common LTC strategies.
Figure 1. This Sandia National Laboratory graph found on Google displays the soot formation zone and NOx formation zone as they relate to flame temperature and equivalence ratio.
Homogeneous Charge Compression Ignition
HCCI is a marriage of traditional diesel (stratified charge compression ignition) and gasoline (homogeneous charge spark ignition) engines. With HCCI, fuel is entrained into the incoming air well before the combustion event. This early injection allows the fuel to homogeneously mix with the air as it is being compressed, as in a port-fuel injected gasoline engine. However, instead of relying on a spark to ignite the mixture, HCCI relies on the heat of compression to ignite the mixture (see Figure 2). The mixture burns simultaneously throughout without the hot flame front of spark ignition combustion or the locally rich flame front of compression ignition combustion.
Figure 2. The Differences Between Diesel, Gasoline, and HCCI Engines
Because there is no explicit control over the combustion event in an HCCI engine, various conditions that induce combustion can be adjusted throughout the engine cycle. Gas temperature, compression ratio, residual exhaust gas, and air/fuel ratio can all be adjusted to attain desired combustion behavior. Data acquisition, processing, and actuation must happen rapidly and in real time for the combustion events between each engine cycle to be adjusted. The propensity for destructive knock in all but the lightest load conditions as well as the difficulty in controlling combustion phasing have pushed researchers to alternate LTC modes.
Premixed Charge Compression Ignition (PCCI)
PCCI, also called partially premixed charge compression ignition (PPCI), is a variant of HCCI that aims to provide some control over the combustion event by injecting a diesel-like late fuel pulse in the compression stroke that dictates the onset of ignition. Ignition still occurs without the aid of a spark plug. The intake air is premixed with fuel before this late fuel injection pulse occurs (hence the name). The early fuel intake stratifies with the air in the cylinder, creating HCCI-like conditions as the compression stroke nears top dead center (TDC). At or near TDC, the late fuel pulse is injected directly into the cylinder. The fuel-rich area of the late injection pulse burns before the fuel-lean homogeneous charge burns. This variable fuel/air mix attempts to control combustion phasing by having the combustion occur over a longer time period than the instantaneous HCCI combustion. The late fuel injection also allows more direct control over where and how the combustion begins in the cylinder, similar to spray-guided direct injection (SGDI) engines. PCCI engines may incorporate both port fuel injectors (PFIs) and direct injectors (DIs).
Reactivity-Controlled Compression Ignition
As an evolution of HCCI and PCCI, RCCI aims to gain more control over combustion phasing by using multiple fuels of differing reactivities in a compression ignition engine. Multiple injections of these various fuels at scheduled intervals provide control over the reactivity of the charge in the cylinder for optimal combustion duration and magnitude. A relatively low reactivity fuel is injected early in the engine cycle and mixes homogeneously with the air. Later in the cycle, a higher reactivity fuel is injected into the cylinder. This approach creates pockets of differing air/fuel ratios and reactivities, causing the combustion to occur at different times and at different rates. Multiple injection systems are required to run an RCCI engine. Permutations of this approach have been demonstrated with all combinations of gasoline, diesel, and natural gas as well as several biofuels and boutique fuels.
Spark Assisted HCCI and HEDGE
HCCI can be very difficult to control under transient conditions; as such, it is often paired with a traditional spark ignition regime to achieve drivability in a transient environment. A spark assisted HCCI (SA-HCCI) engine is one that rapidly transitions between HCCI and spark ignition depending on engine speed and load. For light-load operation, HCCI provides the highest efficiency and lowest emissions. But when the engine is placed under heavy load, cylinder pressure and temperature rapidly change, and it becomes difficult to control HCCI. Destructive engine knock can occur when ignition happens too early and quickly. It is under these transient conditions that the engine switches to a traditional spark-ignited regime, for which the onset of ignition is determined by the timing of the spark event.
Efforts are also being made to improve the traditional gasoline engine. The High-Efficiency Dilute Gasoline Engine (HEDGE) Consortium is a group of researchers who want to improve the gasoline engine to make it a more competitive platform in the future amidst all of the other research engines. HEDGE research currently focuses on knock-resistant combustion chambers, high-energy ignition approaches, and increases in high brake mean effective pressure (BMEP). One of the key elements to achieving these goals is heavy use of exhaust gas recirculation (EGR).
2. Subsystems Required to Control Low Temperature Combustion Engines
Though all of the LTC engine strategies listed above differ in their combinations of fuel and injection timing, they share some common attributes. First, they all demand real-time data acquisition and processing of in-cylinder events so that engine parameters can be tuned to achieve desired results. Many LTC engines accomplish this with cylinder pressure sensors that accurately provide detailed data on combustion phasing, peak pressures, temperature, and so on. Second, they all have precise control over fuel delivery to the cylinder. Some use multipulse injection systems. Some combine PFI and DI. Third, they all maintain some control over combustion temperature and phasing by either trapping exhaust gas from a previous cycle or by introducing exhaust gas back into the cylinder via an external path EGR. Finally, some LTC variants may use spark plugs to assist with the onset of ignition especially under high load.
Many aftermarket engine control units (ECUs) do not offer the power electronics, data acquisition capabilities, or processing speed necessary to control all of the actuators and systems on an LTC engine. Many researchers use multiple ECUs from different manufacturers with fixed personalities to control a single engine. Combustion analysis and engine control typically happen on two separate systems, each with its own software package. As such, development time can be high and software configuration can be tedious. Using commercial off-the-shelf (COTS) products from National Instruments and Drivven, a single system can be built that performs combustion analysis and executes real-time control over each of the subsystems required for an LTC engine to run. Below is an explanation of how National Instruments products interface with each of these engine subsystems.
Real-Time In-Cylinder Pressure Analysis
Pressure sensors inside a cylinder provide the data necessary for precise analysis of combustion phasing as well as the insight needed on adverse combustion conditions like knock and misfire. Using cylinder pressure data, calculations can be performed that determine peak pressure and location in crank angle degrees (CAD), maximum rate of pressure rise and location in CAD, indicated mean effective pressure (IMEP), net MEP, pumping MEP, heat release rate, fuel mass fraction burned (MFB) percentages, and the location of the 50 percent burn point (CA50).
Figure 3. A Common Cylinder Pressure Sensor
MFB calculations are especially interesting in LTC engines because they communicate combustion phasing information—a phenomenon that is not easily controlled in an engine with a complex air/fuel/EGR mixture and possible indirect ignition control. To get an accurate view of combustion phasing inside the cylinder, data must be sampled and processed very quickly and with a high degree of determinism. One control approach is called next-cycle control. In this approach, data must be guaranteed to be available to the ECU’s feedback control algorithms in time to be processed before initiating the next combustion event to adjust cylinder actuators for desired combustion conditions (click here for more information on next-cycle control). To ensure the deterministic calculation of pressure data, a PXI embedded controller with a real-time operating system (Figure 4) is used to run an engine synchronous processing loop at less than 1 ms. This loop gathers data from a simultaneous sampling NI S Series multifunction data acquisition device. S Series devices have up to eight analog input channels and can sample at rates up to 500 kS/s per channel, which is enough to sample cylinder pressure data at 0.1 CAD per cylinder at over 8000 rpm. High-throughput PXI embedded controllers feature multicore, multiple-gigahertz processors that are capable of streaming large amounts of cylinder pressure data and rapidly performing calculations. These deterministic calculations are passed to control loops that actuate fuel injectors, EGR valves, spark plugs, and so on for next-cycle control.
Figure 4. NI PXI-8109 RT Embedded Controller Featuring a 2.66 GHz Intel i7 Processor
An NI R Series field-programmable gate array (FPGA) device is used for reading cam and crankshaft encoder signals with 25 ns resolution. This data is used to phase-align the cylinder pressure data to the engine position with 0.1 CAD of precision. The R Series device is also used for instantaneous engine speed signals and processes all the fuel, spark, and other actuator commands, ensuring they are executed at the precise time required. The R Series FPGA device can also be used for same-cycle control as a combustion regime where pressure data is calculated and processed rapidly enough to influence combustion during a single event.
LTC engines require precise and accurate control over fuel injection timing. In the case of PCCI and RCCI, multipulse fuel injections are timed such that they initiate combustion at a scheduled CAD. Slightly early injection could result in pre-ignition (knock) and slightly late injection could result in unburned fuel. C Series modules by National Instruments and Drivven used with an R Series FPGA device provide the flexibility and control necessary to trigger LTC engine fuel injectors down to 0.1 CAD of resolution. The Drivven C Series line includes the PFI driver module and the DI driver module. They contain all of the power electronics necessary to drive various fuel injectors, including piezoelectric injectors, and provide features such as fully customizable injection timing and adjustable peak and hold currents. The DI and PFI driver modules enable multi-injection scheduling at any crank angle. The FPGA on the R Series device enables same-cycle control of fuel injection through the DI driver module, meaning that fuel injection profiles can be adjusted in real time as combustion occurs inside the cylinder.
Figure 5. A Common Direct Fuel Injector
Drivven Engine Position Tracking (EPT) software is a set of FPGA IP packaged for use in the NI LabVIEW graphical programming environment that tracks the angular position of a wide variety of crankshaft trigger patterns within the LabVIEW FPGA environment. EPT software is designed to supervise fuel and spark output functions based on the engine position calculations done on the FPGA so that the CPU is required to respond to little or no engine synchronous events. Once initialized, EPT software, combined with any number of fuel and/or spark functions, is capable of performing its functions without demanding precious CPU processing time.
Exhaust Gas Recirculation
EGR is the process of introducing exhaust gas into the fresh air, diluting the air and fuel charge, and lowering the combustion temperature. LTC engines typically use a large amount of EGR (>20 percent) to help keep combustion temperatures low and to slow combustion. Exhaust gas is introduced into the cylinder in a variety of ways; some engines use variable valve actuation (VVA) systems on the exhaust valves to “trap” exhaust gas from a previous combustion event (sometimes referred to as “trapped residual”). Others use electronically controlled valves or throttles off the exhaust stream through low-pressure or high-pressure loops to regulate exhaust gas flow into the intake manifold (see Figure 6, courtesy of Renault).
Figure 6. High-Pressure and Low-Pressure EGR Diagram, Courtesy of Renault, image found on Google
For engines that use VVA, the Drivven AD Combo Module measures the cam, crank, and other analog signals to determine engine position, speed, load, and/or air mass. The Drivven AD Combo Module offers a set of automotive-style analog and digital inputs to interface with standard automotive sensors, including 21 analog input channels, two variable reluctance sensor inputs, and two Hall effect sensor or general-purpose digital input channels. EPT software is used to track intake and exhaust valve phases, and the Drivven Port Fuel Injector(PFI)/Low-Side Driver Module , which features four channels of 0 to 100 percent duty-cycle PWM capability, is used to actuate the oil control valve on the VVA cam phaser. The PFI/Low-Side Driver Module is capable of phasing the exhaust valve such that it closes early, trapping residual exhaust gases in the cylinder to foul the next intake charge.
Figure 7. The Drivven CompactRIO PFI/Low-Side Driver Module has four 0 to 100 percent duty-cycle PWM channels and four low or high impedance PFI driver channels.
Low- and high-pressure loops incorporate either a traditional throttle or an electronically controlled valve to apportion exhaust gas into the intake. The Drivven AD Combo Module is used to measure pressure and/or temperature at various points throughout the loop. For throttled engines, the Drivven Electronic Throttle Driver Module has two H bridge circuits and two analog input channels for throttle position feedback. Other EGR systems use the Drivven PFI/Low-Side Driver Module to actuate their electronic EGR valves.
Some LTC strategies work well only under light load and near steady-state conditions; as such, they switch to a conventional spark-ignited regime when the engine is under high load or transient conditions. The Drivven Engine Synchronous TTL (ESTTL)Output Module has the capacity to drive eight smart coils. It is flexible enough to schedule spark events at any crank angle with 0.1 CAD of precision and features 200 nanoseconds of resolution. Engineers can combine multiple ESTTL modules for applications that require more than eight spark plugs.
Figure 8. The Drivven CompactRIO ESTTL Module has eight TTL output channels that fire engine-synchronously for driving smart coils.
3. Other Research Engines
While this document focuses primarily on LTC engines, engine control products from National Instruments and Drivven are flexible enough to be used in numerous other internal combustion engine designs. The open software platform based on the NI LabVIEW FPGA Module enables full customization of actuation timing, and the modular NI CompactRIO platform offers numerous hardware configurations for different application requirements. Applications of free piston, negative valve overlap, spray-guided direct injection (SGDI), dual-fuel, turbocharged, and other engine concepts have all been controlled with engine control products from NI and Drivven.
|LTC Subsystem||Typical Subsystem Components||Required Hardware|
|Real-time pressure Analysis||In-cylinder or non-intrusive pressure sensors, cam and crank signals for engine position tracking||PXI embedded controller with RTOS, NI S Series DAQ device, NI R Series FPGA device|
|Fuel delivery||Piezoelectric or solenoid fuel injectors, common-rail diesel fuel system||NI R Series FPGA device, Drivven DI and/or PFI/Low-Side Driver Module|
|Exhaust gas recirculation||Variable valve actuation (VVA) or electronic throttles or valves||Drivven AD Combo Module, Drivven PFI/Low-Side Driver Module, Drivven Electronic Throttle Controller Module|
|Spark Ignition||Spark plugs||Drivven Engine Synchronous TTL (ESTTL) Output Module|
Table 1. Overview of Required Hardware for LTC Engine Subsystems
While LTC engines promise increased efficiency and low emissions for internal combustion engines, controlling the combusting events inside the cylinders can be a difficult challenge to overcome. With the advance of processor and FPGA technology, LTC and other engine strategies are being controlled and researched in major universities and laboratories across the world. The engine control product line by NI and Drivven offers engineers and researchers the tools they need to get their research engines up and running.
For more information or for a free consultation on your engine control application, contact Drivven at (210) 248-9308 or email@example.com.