The Geneva Motor Show Engineering Essay

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Last March at the Geneva Motor Show, Lotus announced its new Omnivore research engine. The Omnivore is a two-stroke direct injected engine designed to take advantage of the latest in electronic engine management to allow it to run on just about any liquid fuel. In the time since the initial announcement, the boffins at Lotus Engineering have been exercising their creation on the dyno stand to evaluate the performance. Lotus claims the Omnivore is ideally suited to flex-fuel operation with a higher degree of optimization than is possible with existing four stroke engines. The variable compression ratio is achieved by the use of a puck at the top of the combustion chamber. This simple, yet effective system moves up and down affecting the change in geometric compression depending on the load demands on the engine.

The single-cylinder engine uses an air-assisted direct injection system. A movable "puck" in the top of the cylinder head allows the compression ratio to be varied.

The motor uses the Orbital FlexDI fuel injection system which produces fine in-cylinder fuel preparation irrespective of fuel type, and together with air pre-mixing allows efficient two-stroke combustion and low-temperature starting, whilst offering singular opportunity for advanced HCCI control. The engine has so far been run on gasoline and in both spark ignition and homogeneous charge compression ignition (HCCI) modes. The HCCI mode is of particular interest because it is capable of providing diesel engine-like efficiency without the particulate and NOx emissions that require expensive after-treatment systems in a standard diesel engine. Lotus is claiming the Omnivore can operate in HCCI mode in a wide variety of operating conditions and even from a cold start, something that has been problematic for previous HCCI engines. According to the initial test results, the Omnivore is achieving up to a 10 percent improvement in efficiency compared to existing spark ignition direct injected engines.

The Omnivore program is another step in Lotus' research into the combustion processes involved in running an engine on mixtures of alcohol based fuels and gasoline, which included the Lotus Exige 270E Tri-fuel, unveiled at the International Geneva Motor Show in 2008

there is clearly a lot of work yet to do as we are working on trapping valve in this project.

What are 2-stroke engine?

In two-stroke engine cycle engines the cycle of operation is completed in two stroke only and each outward stroke of the piston is power or expansion stroke. The engine piston needs only to compress the fresh charge and to expand the products of combustion, such operation is made possible by the fact that the pumping function in not carried out in the working cylinders but it accomplished either in a separate mechanism called the scavenging pump or in an enclosed crankcase with the back of the engine piston being used as a scavenging pump. The fresh charge is supplied to the engine cylinder at a high enough pressure to displace the burned gases from the previous cycle. The operation of clearing the exhaust gases from the cylinder and filling it more or less completely with fresh charge is called scavenging. The process of scavenging includes both the intake and the exhaust processes.

Many two stroke engines use the piston as a slide valve in conjunction with the inlet and exhaust ports on the side of the cylinder. This arrangement greatly simplifies the mechanical construction of the engine. Very large marine engines and very small reciprocating piston engines are two-stroke engines.

is an internal combustion engine that completes the thermodynamic cycle in two movements of the piston (compared to twice that number for a four-stroke engine). This increased efficiency is accomplished by using the beginning of the compression stroke and the end of the combustion stroke to perform simultaneously the intake and exhaust functions. In this way two-stroke engines often provide strikingly high specific power. Gasoline (spark ignition) versions are particularly useful in lightweight applications such as chainsaws and the concept is also used in diesel compression ignition engines in large and non-weight sensitive applications such as ships and locomotives.

Problems of the two-stroke engine

actually the two-stroke engine should perform twice the performance of a four-stroke engine with the same cubic capacity. Though it is just possible to gain a performance that is about 50% better. The reasons are obvious: The cylinder can't be filled up with the same amount of fuel as in the four-stroke engine, because the individual strokes are separated not so clearly. If more fuel is induced, it leaves the combustion chamber through the ejection pipe without being burnt. Many concepts were developed to provide a better expulsion of the exhaust in way that the fresh gas doesn't leave the combustion chamber. Though all these inventions, the filling of the two-stroke engine is always worse than in the four-stroke engine, which loses fresh fuel only because of the "overlap" of the valve times (both valves are open for an instant). Beside these performance-technical problems, there are also increasing difficulties with the environment. The fuel mixture of the two-stroke engine often gets shifted with a certain quantity of oil because of the necessary lubrication. Unfortunately the oil gets burnt partly, too, and harmful gases are expulsed by the engine.

Gasoline direct injection:

In internal combustion engines, Gasoline Direct Injection (GDI), sometimes known as Fuel Stratified Injection (FSI), is an increasingly popular type of fuel injection system employed in modern four and two-stroke petrol engines. The petrol/gasoline is highly pressurised, and injected by high voltage driven injectors via a common rail fuel line directly into the combustion chamber of each cylinder, as opposed to conventional single or multi-point fuel injection that happens in the intake manifold tract, or cylinder port. In some applications, gasoline direct injection enables a stratified fuel charge (ultra lean burn) combustion for improved fuel efficiency, and reduced emission levels at low load.

Basic theory of operation:

The major advantages of a GDI engine are lower emission levels, increased fuel efficiency and higher engine power output. In addition, the cooling effect of the injected fuel and the more evenly dispersed combustion mixtures and temperatures allow for improved ignition timing settings which are an equally important system requirement.

Emissions levels can be more accurately controlled with the GDI system. The lower levels are achieved by the precise control over the amount of fuel, air and ignition settings which are varied according to the engine load conditions and ambient air temperatures.

In addition, there are no throttling losses in some

GDI designed engines, when compared to a conventional fuel injected or carbureted engine, which greatly improves efficiency and reduces 'pumping losses' in engines without a throttle plate. Engine speed is controlled by the engine management system which regulates fuel injection and ignition timing parameters, instead of having a throttle plate which restricts the incoming air supply. Adding this function to the engine management system requires considerable enhancement of its processing and memory, as direct injection plus other engine management systems must have very precise mapping for good performance and driveability.

The engine management system continually chooses among three combustion

cycles: ultra lean burn, stoichiometric, and full power output. Each cycle is characterised by the air-fuel ratio. The stoichiometric air-fuel ratio for petrol (gasoline) engines is 14.7:1 by weight, but the ultra lean cycle can involve ratios as high as 35:1 (or even higher in some engines, for very limited periods). These mixtures are much leaner than in a conventional fuel injected engine and reduce fuel consumption and certain levels of exhaust emissions considerably.

Direct injection is supported by other engine management systems such as variable valve timing (VVT) with variable length intake manifold (VLIM) or acoustic controlled intake system (ACIS). A high performance exhaust gas recirculation valve (EGR) will almost certainly be required to reduce the high nitrogen oxides (NOx) emissions which will result from burning ultra lean mixtures.

Conventional fuel injection engines could inject fuel throughout the 4 stroke sequence, as the injector injects fuel onto the back of a closed valve. Earlier direct injection engines, where the injector injects fuel directly into the cylinder, were limited to the induction stroke of the piston.

As the RPM increases, the time available to inject fuel decreases. Newer GDI systems have sufficient fuel pressure to inject more than once during a single cycle. Fuel injection takes place in two phases. During the intake stroke, some amount of fuel is "pre-injected" into the combustion chamber which cools the incoming air, thus improving volumetric efficiency and ensuring an even fuel/air mixture within the combustion chamber. Main injection takes place as the piston approaches top dead centre on the compression stroke, shortly before ignition.

Two-stroke engines:

The benefits of direct injection are even more pronounced in two-stroke engines, because it eliminates much of the pollution that their conventional design causes. In conventional two-strokes, the exhaust and intake ports are both open at the same time, at the bottom of the piston stroke. A large portion of the fuel/air mixture entering the cylinder from the crankcase through the intake ports goes directly out, unburned, through the exhaust port. With direct injection, only air comes from the crankcase, and fuel is not injected until the piston rises and all ports are closed.

HCCI engine:

What is HCCI?

HCCI is an alternative piston-engine combustion process that can provide efficiencies as high ascompression-ignition, direct-injection (CIDI) engines (an advanced version of the commonly known diesel engine) while, unlike CIDI engines, producing ultra-low oxides of nitrogen (NOx) and particulate matter (PM) emissions. HCCI engines operate on the principle of having a dilute,premixed charge that reacts and burns volumetrically throughout the cylinder as it is compressed by the piston. In some regards, HCCI incorporates the best features of both spark ignition (SI) and compression ignition (CI), as shown in Figure 1. As in an SI engine, the charge is well mixed, which minimizes particulate emissions, and as in a CIDI engine, the charge is compression ignited

and has no throttling losses, which leads to high efficiency. However, unlike either of these

conventional engines, the combustion occurs simultaneously throughout the volume rather than in a flame front. This important attribute of HCCI allows combustion to occur at much lower temperatures, dramatically reducing engine-out emissions of NOx.

Most engines employing HCCI to date have dual mode combustion systems in which traditional SI or CI combustion is used for operating conditions where HCCI operation is more difficult.

Typically, the engine is cold-started as an SI or CIDI engine, then switched to HCCI mode for idle and low- to mid-load operation to obtain the benefits of HCCI in this regime, which comprises a large portion of typical automotive driving cycles. For high-load operation, the engine would again be switched to SI or CIDI operation. Research efforts are underway to extend the range of HCCI operation, as discussed in the body of this report.

What are the Advantages of HCCI?

The advantages of HCCI are numerous and depend on the combustion system to which it is

compared. Relative to SI gasoline engines, HCCI engines are more efficient, approaching the

efficiency of a CIDI engine. This improved efficiency results from three sources: the elimination of throttling losses, the use of high compression ratios (similar to a CIDI engine), and a shorter combustion duration (since it is not necessary for a flame to propagate across the cylinder). HCCI engines also have lower engine-out NOx than SI engines. Although three-way catalysts are adequate for removing NOx from current-technology SI engine exhaust, low NOx is an important advantage relative to spark-ignition, direct-injection (SIDI) technology, which is being considered for future SI engines.

Relative to CIDI engines, HCCI engines have substantially lower emissions of PM and NOx.

(Emissions of PM and NOx are the major impediments to CIDI engines meeting future emissions standards and are the focus of extensive current research.) The low emissions of PM and NOx in HCCI engines are a result of the dilute homogeneous air and fuel mixture in addition to low combustion temperatures. The charge in an HCCI engine may be made dilute by being very lean, by stratification, by using exhaust gas recirculation (EGR), or some combination of these. Because flame propagation is not required, dilution levels can be much higher than the levels tolerated by either SI or CIDI engines. Combustion is induced throughout the charge volume by compression heating due to the piston motion, and it will occur in almost any fuel/air/exhaust-gas mixture once the 800 to 1100 K ignition temperature (depending on the type of fuel) is reached. In contrast, in typical CI engines, minimum flame temperatures are 1900 to 2100 K, high enough to make unacceptable levels of NOx. Additionally, the combustion duration in HCCI engines is much shorter than in CIDI engines since it is not limited by the rate of fuel/air mixing. This shorter combustion duration gives the HCCI engine an efficiency advantage. Finally, HCCI engines may be lower cost than CIDI engines since they would likely use lower-pressure fuel-injection equipment.

Another advantage of HCCI combustion is its fuel-flexibility. HCCI operation has been shown using a wide range of fuels. Gasoline is particularly well suited for HCCI operation. Highly efficient CIDI engines, on the other hand, cannot run on gasoline due to its low cetane number. With successful R&D, HCCI engines might be commercialized in light-duty passenger vehicles by 2010, and by 2015 as much as a half-million barrels of oil per day may be saved. Tests have also shown that under optimized conditions HCCI combustion can be very repeatable, resulting in smooth engine operation. The emission control systems for HCCI engines have the potential to be less costly and less dependent on scarce precious metals than either SI or CIDI engines.

HCCI is potentially applicable to both automobile and heavy truck engines. In fact, it could be scaled to virtually every size-class of transportation engines from small motorcycle to large ship engines. HCCI is also applicable to piston engines used outside the transportation sector such as those used for electrical power generation and pipeline pumping.

Results Using Different Fuels

One of the advantages of HCCI combustion is its intrinsic fuel flexibility. HCCI combustion has little sensitivity to fuel characteristics such as lubricity and laminar flame speed. Fuels with any octane or cetane number can be burned, although the operating conditions must be adjusted to accommodate different fuels, which can impact efficiency, as discussed below. An HCCI engine with VCR or VVT could, in principle, operate on any hydrocarbon or alcohol liquid fuel, as long as the fuel is vaporized and mixed with the air before ignition.

The literature shows that HCCI has been achieved with multiple fuels. The main fuels that have been used are gasoline, diesel fuel, propane, natural gas, and single- and dual-component mixtures of the gasoline and diesel primary reference fuels (iso-octane and n-heptane, respectively). The applicability of these fuels to HCCI engines is discussed below. Other fuels (methanol, ethanol, acetone) have also been tried in experiments, but with inconclusive results.

Gasoline: Gasoline has multiple advantages as an HCCI fuel. Gasoline also has a high octane number (87 to 92 in the U.S. and up to 98 in Europe), which allows the use of reasonably high compression ratios in HCCI engines. Actual compression ratios for gasoline-fueled HCCI engine data vary from 12:1 to 21:1 depending on the fuel octane number, intake air temperature, and the specific engine used (which may affect the amount of hot residual naturally retained). This compression-ratio range allows gasoline-fueled HCCI engines to achieve relatively high thermal efficiencies (in the range of diesel-fueled CIDI engine efficiencies). A potential drawback to higher compression ratios is that the engine design must accommodate the relatively high cylinder pressures that can result, particularly at high engine loads (see discussion in Section V B). Additional advantages of gasoline include easy evaporation, simple mixture preparation, and a ubiquitous refueling infrastructure.

Diesel Fuel: Diesel fuel autoignites rapidly at relatively low temperatures but is difficult to

evaporate. To obtain diesel-fuel HCCI combustion, the air-fuel mixture must be heated

considerably to evaporate the fuel. The compression ratio of the engine must be very low (8:1

or lower) to obtain satisfactory combustion, which results in a low engine efficiency.

Alternatively, the fuel can be injected in-cylinder, but without air preheating, temperatures are not sufficiently high for diesel-fuel vaporization until well up the compression stroke. This strategy often results in incomplete fuel vaporization and poor mixture preparation, which can lead to particulate matter and NOx emissions

Refrences:

http://www-erd.llnl.gov/FuelsoftheFuture/pdf_files/hccirtc.pdf

Fundamentals of internal combustion engines. author: H.N.gupta

http://www.leftlanenews.com/lotus-omnivore.html

http://green.autoblog.com/2009/12/10/lotus-omnivore-engine-is-already-10-more-efficient-with-more/

http://www.motor.org.uk/documentlibrary/Tech%20Talk/MIM%20TECH%20TALK%20JUNE%202010.pdf

http://www-erd.llnl.gov/FuelsoftheFuture/pdf_files/hccirtc.pdf

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