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TIAX to Develop Sensor Technology for Low-Temperature Combustion Engines

11 October 2006

The US Department of Energy (DOE) has awarded a $400,000 contract to TIAX, a leading collaborative product and technology development firm, to develop sensor technology to help control the start of combustion (SOC) in low-temperature combustion (LTC) engines.

DOE has set a goal for 2012 of developing the understanding of novel low-temperature engine combustion regimes needed to increase light-duty engine efficiency from today’s 30% to 45%. Heavy-duty engines have an efficiency target of 55%.

LTC engines, of which Homogeneous Charge Compression Ignition (HCCI) engines are an example, promise diesel-like efficiency (high compression ratios and no throttling) combined with low engine-out NOx and low particulate emissions. (Honda’s promised Tier 2 Bin 5 diesel uses PCCI, a variant of LTC. Earlier post.)

Such combustion regimes are complex to manage, however, and face a number of barriers, including control of combustion and heat-release rates for steady and transient conditions. A number of initiatives are exploring the use of a variety of mechanisms, including mixture formulations, injection strategies (early, late, close-coupled pilot and post, multiple main), valve timings, and spray geometries as mechanisms to optimize combustion under a variety of load ranges. Knowing when combustion initiates under varying conditions could be an important element for adaptive combustion control.

As the prime contractor, TIAX will team with Wayne State University, a leader in engine diagnostics and control, on advancing sensor technology to address this issue. In earlier work for the DOE, TIAX developed a non-intrusive microphone sensor mounted on the engine block that is designed to determine the start of combustion on a cycle-by-cycle basis in the same way that a stethoscope senses heart beats.

The creation of a durable and effective start-of-combustion sensor could solve one of the most critical challenges to the viability of high-efficiency automotive engine platforms. We believe that the sensor technology that we are developing will enable a new breed of engines that can save 10 to 15 percent of the US petroleum now used in transportation, while meeting or exceeding 2010 emissions targets.

—Kenan Sahin, CEO and Founder of TIAX

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October 11, 2006 in Engines, Fuel Efficiency | Permalink | Comments (21) | TrackBack (0)

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The term low-temperature combustion strikes me as quite confusing and misleading. Perhaps reduced-temperature combustion (RTC) would be a more accurate way to underline the low-NOx potential of HCCI.

The basis for my nitpick is that the combustion of certain fuel compounds (e.g. long-chained alkanes, the primary constituent of gasoil and kerosene) exhibits what is called a negative temperature coefficient (NTC) at intermediate temperatures. The rate of heat release (ROHR) increases at low temperatures, falls again in the NTC range before rising rapidly once more at high temperatures. The ROHR remains positive throughout combustion, but its rise is not monotonous.

NTC behavior occurs because combustion is actually an incredibly complicated process involving thousands of intermediate chemical species. Each individual chemical reaction occurs simultaneously in both directions, typically yielding an exothermal result. Counterintuitively, some of these reactions become endothermal in the NTC temperature range. The observed global ROHR at a given temperature is the sum of all local heat release and absorption events.

The NTC feature of gasoil fuel is at the heart of why conventional compression ignition works at all: it buys time for the fuel-air mixture to form even as precursor reactions are already in progress. The process is imperfect because the NTC effect is too small by itself; additional delay is feasible using moderate rates of externally cooled EGR. This also reduces peak local temperatures and hence, NOx production. Unfortunately, although the improved mixture formation cuts down on PM production, less of what remains is post-combusted in the diffusion flame fronts. The upshot is that conventional CI with low EGR rates trades off lower engine-out NOx against higher PM levels.

By contrast, short-chained alkanes (the primary consituent of gasoline) exhibit much weaker or no NTC behavior. High-octane fuels are hard to ignite but once they do, combustion is very rapid. That is why a (typically homogenous) air-fuel mixture is prepared prior to firing the spark plug at just the right moment. Uncontrolled auto ignition (engine knock) can occur if the compression ratio or the initial temperature are too high. In severe cases, this can very quickly lead to total engine failure.

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In HCCI combustion, the objective is to control the process such that controlled auto-ignition occurs almost simultaneously throughout the chamber. A prerequsite for accurate ignition timing is cycle-specific information about the oxygen content of the charge at intake valve close. Moreover, the charge must be well pre-mixed, which coincidentally avoids the production of particulate matter. Finally, no flame front is formed, so local temperatures stay below the range (>2000 deg C) at which nitrous oxides are produced. Thermodynamically, HCCI approximates isochoric combustion, which is very efficient. Unfortunately, it also generates high combustion noise and a pressure spike that leads to high mechanical stresses. These drawbacks limit HCCI to low part load operation; during warm-up, extended idling and for high torque and/or power, conventional combustion must be used.

Note that there are countless variations on the basic HCCI concept, each involving a slightly different control strategy. The resulting acronym jungle keeps patent lawyers in gainful employment. I'm using HCCI here as a catch-all term for all intermittent combustion strategies that do not involve a flame front.

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For fuels exhibiting NTC behavior, researchers have discovered that a single late injection just before TDC works well enough, provided the compressed gas contains a high proportion of inert compounds (high rate of externally cooled EGR). Ironically, with high-quality gasoil (high cetane number) the EGR rate has to be higher to slow down the precursor reactions sufficiently for mixture homogenization to occur. At elevated loads, the injection even has to be postponed until *after* TDC to avoid excessive pressures. Injection timing and volume is fine-tuned using fast closed-loop control based on an in-cylinder pressure sensor, which may be integrated with the glow plug.

For fuels without pronounced NTC behavior, the initial charge temperature must be carefully controlled to ensure the ROHR curve (cp. pressure trace) is properly correlated with the crankshaft angle. Even a one degree shift either way from the optimum can reduce power output by several percent. At 3000 RPM, 1 CAD is traversed in just 55 microseconds. Injection volume is fine-tuned using fast closed-loop control based on ion current measurements. For these, the spark plug is very briefly operated at below-ignition current levels; special high-voltage signal processing circuitry is requried.

Both types of in-cylinder sensor already exist, but the price tag has so far limited their application to R&D work. The sensor TIAX is developing presumably relies on cheaper transducer technology and fancier signal processing, though the above article only hints that it might be based on acoustic signals (cp. standard knock sensors).

Thanks for the post Rafael. Would you explain why an engine can't be strengthened to withstand the mechanical stresses of the HCCI process, when they can withstand diesel stresses? I understand that warm-up is a separate issue. If the regimen can indeed only be used for say 20 minutes of a 40 minute commute, what real advantage will be realized, considering that catalysts will still be needed for the rest of the trip. I'm also unsure of the advantage of HCCI over a Euro 6 diesel, other than requiring less post-processing of the exhaust. Wouldn't the cost of the HCCI's new sensors, ignition, and computers outweigh the potential savings?

JC -

the upper torque limit for HCCI operation is defined not by mechanical strength but by combustion noise levels and, especially, by engine-out NOx emissions.

Interior noise can be deadened by it's harder to achieve for exterior noise emissions, which are limited by law in Europe. Engines over here tend to be noisier because they tend to be smaller and revv higher than those of similarly-sized vehicles in the US. Moreover, cars are, on average, smaller and lighter to begin with and, pedestrians tend to be closer to traffic.

Wrt emissions, increasing torque in a given engine setup requires reducing the EGR rate. As described above, this accelerates the combustion event and increases peak temperatures. Beyond a certain point, HCCI is no longer any better than conventional combustion.

One possibility would be to design a large-displacement engine that will simply never be operated at high specific torque or power loads. Unfortunately, HCCI tops out at just ~4 bar bmep in NA and ~6-8 bar in turbocharged engines, at around 3000 RPM. Beyond about that speed, the area of stable HCCI operation is approximately limited by specific power. You'd be lucky to achieve 15kW/l out of a four-stroke turbodiesel in HCCI mode, about 1/4 of the value feasible with conventional CI.

To get e.g. 90kW rated power for a regular C-segment vehicle, you'd have to fit an engine with 6 liters of displacement under the hood! Even allowing for 750cc per cylinder on account of the low RPM limit, you'd still need a V8 or something truly exotic, like an opposed-piston design or a two-stroke diesel with uniflow scavenging. Afaik, no carmaker is contemplating an HCCI-only engine for a production vehicle.

There are many ways of theoretically acheiving HCCI.
One that has got my interest is the Compression Ignition by Air Injection.
http://kitkat.wvu.edu:8080/files/4501/Echavarria_F_dissertation.pdf
Abstract


Compression ignition by air injection (CIBAI) has been successfully achieved in a modified single cylinder, four-stroke, spark ignition cooperative fuel research (CFR) engine. The CIBAI cycle was invented by Professor John Loth and Professor Gary Morris, US patent No's: 6,994,057 Feb. 7, 2006 and 6,899,061 May 31, 2005. This new revolutionary combustion concept has the potential to become an alternative to traditional (SI) spark ignited and compression ignited (CI) diesel engines. A CIBAI engine consists of two or more even numbers of adjacent cylinders that work in synchronization. One cylinder normally contains a conventional air-fuel mixture at a compression ratio limited by fuel auto-ignition properties while the second cylinder contains air-only at high compression ratio. Only during the compression stroke are these cylinders separated with a closed cylinder-connecting valve (CCV). The CCV valve normally opens near top dead center (TDC) to allow transfer of high-pressure air from the air-only cylinder into the air-fuel mixture cylinder. Mixing air with pre-evaporated fuel with hot high-pressure air causes rapid two-step pressure rise, first by air addition and second by combustion compression. Ignition by air injection provides high ignition energy allowing very lean mixtures to be ignited for low emissions. Expanding combustion gases in both cylinders results in increased expansion ratio and thus thermal efficiency. The objective of this dissertation was to demonstrate experimentally the viability of achieving ignition by air injection (CIBAI) for controlled auto-ignition in a CFR engine. This experimental work involved the development of an air injection model, and the design, assembly, and testing of a highly specialized air injection and timing equipment. These experiments were designed to substitute CIBAI ignition for one cycle in a spark ignition engine. The CIBAI engine cycle analysis is included, followed by an analytical model of the air injection process. A controller for the air injection and timing system was designed, built and tested under different operating conditions until a satisfactory experimental procedure was developed for testing using the CIBAI concept. Based on the measured pressure-time history a numerical modeling code was developed to analyze power and combustion parameters (indicated net work, indicated mean effective pressure (IMEP), net heat release, net heat release rate, mass fraction burned (MFB), temperature history, combustion duration, and ignition delay). Finally, a parametric study was conducted to determine the effect of compression ratio, intake temperature, air-fuel ratio, air charged pressure, and air injection timing on CIBAI combustion. Experimental and numerical model results indicated that ignition is readily achieved by air injection (CIBAI) in a CFR engine using the proper air injection system and proper air injection timing strategy.

The advantage of this being no spark plug and no high pressure fuel system.
Potentially you could use a carburator.

Thanks Rafael and Doug for the insightful info on the subject.
It seems that CIBAI is quite an ingenious approach to HCCI by being able to precisely control the timing of peak pressure and hence the ignition timing just by compression-ignition. The problem with HCCI according to various studies is that ignition occurs too early, before TDC, and that along with the rapid combustion puts a lot of stress on the engine, causing vibration and noise, and prolong exposure of the piston and cylinder head and valves to hot exhaust gas. No matter how "cool" the combustion, the temperature will always exceed the melting points of aluminum alloy.

CIBAI concept enable one to inject high-pressure air into the cylinder head more gradually and in a controlled fashion, causing peak pressure to rise after TDC as the piston is moving down, hence greatly reducing stress on engine piston, conrod, bearing etc, and eliminate the risk of pre-ignition damage as well as misfiring, as well as minimizing exposure time of piston and cylinder head to peak-combustion temperature and pressure. This concept does not need fancy sensors nor sophisticated monitoring system. Since the fuel-air is homogenously premixed, PM is greatly reduced with more complete combustion, and the closer-to-isobaric-heat-addition by combustion occuring on the piston down-going stroke reduces the peak combustion temp, hence reducing the potential for NOx formation. The high expansion volume due to the addition of the air-injection cylinder will help boost the thermal efficiency to make up for the lower peak combustion temperature.

Doug, what is the potential of commercialization of this CIBAI concept and what are the potential disadvantages?

Doug -

CIBAI sounds like a pretty complicated approach to me. AVL List, a global engine R&D consultancy with a staff of ~3400, has already wrapped up work on a multi-cylinder real-world gasoline "CSi" engine and successfully ran the transient NEDC cycle on it in the lab. This includes smooth and robust transitions between HCCI and conventional combustion modes. They're even further along on diesels, as are other companies.

And btw, if you do HCCI right, combustion timing is actually *more* predictable and controllable in stationary operation than with a spark plug. Transients are tougher to control, but those problems were already solved a couple of years ago. Research hasn't stopped by any means but the fundamental hurdles you mention have been cleared.

What has kept HCCI from implementation in series production vehicles is the combined cost of the variable valve train cq. externally cooled EGR infrastrucutre and the in-cylinder sensors. Noise deadening, compute power and engine control software are lesser cost factors. For gasoline, there are cheaper ways to achieve fuel economy improvements (albeit of a lesser order). For diesel, HCCI was hot for a while because US HDVs have to be 85% cleaner by 2010 than they were in 2005 - interest may wane now that regulators are accepting SCR systems, which were introduced in many European HDV models last year.

I enjoyed reading your article on:TIAX to Develop Sensor Technology for Low Temperature Combustion Engines. Besides a complete description of HCCI you wrote the last 2 pages on our newly invented CIBAI cycle engine. This was an abstract of a PhD student research paper, who performed a complex simulation of CIBAI ignition in a CFR engine. This has little to do with the simple CIBAI engine itself. We expect to have a CIBAI engine running within 2 month. This engine is much simpler then either Otto or Diesel engines, see comparisons below
CIBAI has: no throttle valve, no spark plug, no high pressure fuel pump and injectors, but only one rugged extra component, for which we use a 1/2" SS rod with a hole in it, it serves as the two cyclinders connecting valve.

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