Engine Powered by Liquid Hydrogen

Wide Range of Flammability:

In comparing with all other fuels, Hydrogen has a really broad scope of flammability. Hence, Hydrogen can be combusted in an internal burning engine over a broad scope of fuel-air mixtures. One of the important advantages of this is that Hydrogen fuel can run a really thin mixture.

A thin mixture is one in which the sum of fuel is less than the theoretical, stoichiometric or chemically ideal sum needed for burning with a given sum of air. This is why it is reasonably easy to acquire an engine to get down on H.

In general, fuel economic system is greater and the burning reaction is more complete when a vehicle is run on a thin mixture. Besides, the concluding burning temperature is lower hence cut downing the sum of pollutants, such as N oxides, emitted in the fumes. However, there is a bound to how thin the engine can be run, as thin operation can significantly cut down the power end product due to a decrease in the volumetric heating value of the air/fuel mixture

Low Ignition Energy:

Ignition energy is the energy needed to light a fuel. Hydrogen has really low ignition energy. The sum of energy needed to light H is about one order of magnitude less than that required for gasolene. The feature of low Ignition energy enables hydrogen engines to light thin mixtures and besides ensures prompt ignition.

Unfortunately, the low ignition energy besides carries the hazard that hot gases and hot musca volitanss on the cylinder can function as beginnings of ignition and hence making jobs such as premature ignition and flashback. Preventing the above mentioned jobs is one of the jobs associated with runing an engine on H. The broad flammability scope of H agencies that about any mixture can be ignited by a hot topographic point.

Small Quenching Distance:

Hydrogen has a little extinction distance, even smaller than that of gasolene. Hence, it ‘s more hard to slake a H fire than a gasolene fire. The inclination for blowback additions as a consequence of smaller extinction distance, since the fire from a hydrogen-air mixture more readily passes a about closed consumption valve, than a hydrocarbon-air fire.

High Auto-Ignition Temperature:

Hydrogen has a comparatively high auto-ignition temperature. This characteristic is of import because the hydrogen-air mixture has to be compressed and therefore plays an of import function in finding the compaction ratio that an engine can utilize, since the temperature rise during compaction is related to the compaction ratio.

The temperature should non transcend the auto-ignition temperature, as this would do a premature ignition. Hence, the absolute concluding temperature controls the compaction ratio. The high car ignition temperature of H allows larger compaction ratios to be used in a H engine than in a hydrocarbon engine. On the other manus, H is hard to light in a compaction ignition or Diesel constellation, because the temperatures needed for those types of ignition are comparatively high.

High Flame Speed

Hydrogen has high fire velocity at stoichiometric ratios. Under these conditions, the H fire velocity is about an order of magnitude higher ( faster ) than that of gasolene. This means that H engines can more closely approach the thermodynamically ideal engine rhythm. At leaner mixtures, nevertheless, the fire speed decreases significantly.

High Diffusivity

Hydrogen has really high diffusivity. This ability to scatter in air is well greater than gasolene and is advantageous for two chief grounds. First, it facilitates the formation of a unvarying mixture of fuel and air. Second, if a H leak develops, the H disperses quickly. Therefore, insecure conditions can either be avoided or minimized.

Low Density

Hydrogen has really low denseness. This consequences in two jobs when used in an internal burning engine. First, a really big volume is necessary to hive away adequate H to give a vehicle an equal drive scope. Second, the energy denseness of a hydrogen-air mixture, and therefore the power end product, is reduced.

Fuel Delivery Systems

Hydrogen fuel bringing system can be broken down into three chief types:

• Cardinal injection ( or “carburetted” )
• Port injection
• Direct injection

Central and port fuel bringing systems injection forms the fuel-air mixture during the intake shot. In the instance of cardinal injection or a carburetor, the injection is at the recess of the air intake manifold. In the instance of port injection, it is injected at the recess port. Direct cylinder injection is more technologically sophisticated and involves organizing the fuel-air mixture inside the burning cylinder after the air consumption valve has closed.

Cardinal Injection or Carburetted Systems:

The simplest method of presenting fuel to a H engine is by manner of a carburetor or cardinal injection system. This system has advantages for a H engine. First, cardinal injection does non necessitate the H supply force per unit area to be every bit high as for other methods. Second, cardinal injection or carburetors are used on gasolene engines, doing it easy to change over a standard gasolene engine to hydrogen or a gasoline/hydrogen engine.

The disadvantage of cardinal injection is that it is more susceptible to irregular burning due to pre-ignition and back fire. The greater sum of hydrogen/air mixture within the consumption manifold compounds the effects of pre-ignition.

Port Injection Systems

The port injection fuel bringing system injects fuel straight into the consumption manifold at each consumption port, instead than pulling fuel in at a cardinal point. Typically, the H is injected into the manifold after the beginning of the consumption shot. At this point conditions are much less terrible and the chance for premature ignition is reduced. In port injection, the air is injected individually at the beginning of the consumption shot to thin the hot residuary gases and cool any hot musca volitanss. Since less gas ( H or air ) is in the manifold at any one clip, any pre-ignition is less terrible. The recess supply force per unit area for port injection tends to be higher than for carburetted or cardinal injection systems, but less than for direct injection systems. The changeless volume injection ( CVI ) system uses a mechanical cam-operated device to clip the injection of the H to each cylinder. The CVI block is shown on the far right of the exposure with four fuel lines go outing on left side of the block ( one fuel line for each cylinder ) .

The electronic fuel injection ( EFI ) system meters the H to each cylinder. This system uses single electronic fuel injectors ( solenoid valves ) for each cylinder and pumped to a common fuel rail located down the Centre of the consumption manifold. Whereas the CVI system uses changeless injection timing and variable fuel rail force per unit area, the EFI system uses variable injection timing and changeless fuel rail force per unit area.

Direct Injection Systems

More sophisticated H engines use direct injection into the burning cylinder during the compaction shot. In direct injection, the consumption valve is closed when the fuel is injected, wholly avoiding premature ignition during the intake shot. Consequently the engine can non backlash into the consumption manifold. The power end product of a direct injected H engine is 20 % more than for a gasolene engine and 42 % more than a H engine utilizing a carburetor. While direct injection solves the job of pre-ignition in the consumption manifold, it does non needfully forestall pre ignition within the burning chamber. In add-on, due to the reduced commixture clip of the air and fuel in a direct injection engine, the air/fuel mixture can be non-homogenous. Surveies have suggested this can take to higher NOx emanations than the non-direct injection systems. Direct injection systems require a higher fuel rail force per unit area than the other.

Thermal Dilution

Pre-ignition conditions can be curbed utilizing thermic dilution techniques such as fumes gas recirculation ( EGR ) or H2O injection. As the name implies, an EGR system re-circulates a part of the fumes gases back into the consumption manifold. The debut of exhaust gases helps to cut down the temperature of hot musca volitanss, cut downing the possibility of pre-ignition. Additionally, re-circulating fumes gases cut down the peak burning temperature, which reduces NOx emanations. Typically a 25 to 30 % recirculation of fumes gas is effectual in extinguishing blowback. On the other manus, the power end product of the engine is reduced when utilizing EGR. The presence of exhaust gases reduces the sum of fuel mixture that can be drawn into the burning chamber.

Another technique for thermally thining the fuel mixture is the injection of H2O. Injecting H2O into the H watercourse prior to blending with air has produced better consequences than shooting it into the hydrogen-air mixture within the consumption manifold. A possible job with this type of system is that H2O can acquire assorted with the oil, so care must be taken to guarantee that seals do non leak.

Engine Design

The most effectual agencies of commanding pre-ignition and knock is to re-design the engine for H usage, specifically the burning chamber and the chilling system. A discoid burning chamber ( with a level Piston and chamber ceiling ) can be used to cut down turbulency within the chamber. The disc form helps bring forth low radial and digressive speed constituents and does non magnify recess whirl during compaction. Since unburned hydrocarbons are non a concern in H engines, a big bore-to-stroke ratio can be used with this engine. To suit the wider scope of fire velocities that occur over a greater scope of equality ratios, two flicker stoppers are needed. The chilling system must be designed to supply unvarying flow to all locations that need chilling. Extra steps to diminish the chance of pre ignition are the usage of two little exhaust valves as opposed to a individual big one, and the development of an effectual scavenging system, that is, a agency of displacing exhaust gas from the burning chamber with fresh air.

Ignition Systems

Due to hydrogen ‘s low ignition energy bound, lighting H is easy and gasoline ignition systems can be used. At really thin air/fuel ratios ( 130:1 to 180:1 ) the fire speed is reduced well and the usage of a double flicker stopper system is preferred. Ignition systems that use a waste flicker system should non be used for H engines. These systems energize the flicker each clip the Piston is at top dead Centre whether or non the Piston is on the compaction stroke or on its exhaust shot. For gasolene engines, waste flicker systems work good and are less expensive than other systems. For H engines, the waste flickers are a beginning of pre-ignition. Spark stopper for a H engine should hold a cold evaluation and have non-platinum tips. A cold-rated stopper is one that transfers heat from the stopper tip to the cylinder caput quicker than a hot-rated flicker stopper. This means the opportunities of the flicker stopper tip lighting the air/fuel charge is reduced. Hot rated spark stoppers are designed to keep a certain sum of heat so that C sedimentations do non roll up. Since H does non incorporate C, hot-rated flicker stoppers do non function a utile map. Platinum-tip flicker stopper should besides be avoided since Pt is a accelerator, doing H to oxidise with air.

Crankcase Ventilation:

Crankcase airing is even more of import for H engines than for gasolene engines. As with gasolene engines, un-burnt fuel can ooze by the Piston rings and enter the crankcase. Since H has a lower energy ignition bound than gasolene, any un-burnt H come ining the crankcase has a greater opportunity of lighting. Hydrogen should be prevented from roll uping through airing. Ignition within the crankcase can be merely a startling noise or consequence in engine fire. When H ignites within the crankcase, a sudden force per unit area rise occurs. To alleviate this force per unit area, a force per unit area alleviation valve must be installed on the valve screen. Exhaust gases can besides ooze by the Piston rings into the crankcase. Since H fumes is H2O vapor, H2O can distill in the crankcase when proper airing is non provided. The commixture of H2O into the crankcase oil reduces its lubrication ability, ensuing in a higher grade of engine wear.

Emissions:

The burning of H with O green goodss H2O as its lone merchandise:

2H2 + O2 = 2H2O

The burning of H with air nevertheless can besides bring forth oxides of N ( NOx ) :

H2 + O2 + N2 = H2O + N2 + NOx

The oxides of N are created due to the high temperatures generated within the burning chamber during burning. This high temperature causes some of the N in the air to unite with the O in the air. The sum of NOx formed depends on:

• The air/fuel ratio
• The engine compaction ratio
• The engine velocity
• The ignition timing
• Whether thermic dilution is utilised

In add-on to oxides of N, hints of C monoxide and C dioxide can be present in the fumes gas, due to ooze oil combustion in the burning chamber. Depending on the status of the engine ( combustion of oil ) and the operating scheme used ( a rich versus thin air/fuel ratio ) , a H engine can bring forth from about zero emanations ( every bit low as a few ppm ) to high NOx and important C monoxide emanations.

Power End product:

The theoretical maximal power end product from a H engine depends on the air/fuel ratio and fuel injection method used. As mentioned in above, the stoichiometric air/fuel ratio for H is 34:1. At this air/fuel ratio, H will displace 29 % of the burning chamber go forthing merely 71 % for the air. As a consequence, the energy content of this mixture will be less than it would be if the fuel were gasolene ( since gasolene is a liquid, it merely occupies a really little volume of the burning chamber, and therefore allows more air to enter ) . Since both the carburetted and larboard injection methods mix the fuel and air prior to it come ining the burning chamber, these systems limit the maximal theoretical power gettable to about 85 % of that of gasolene engines. For direct injection systems, which mix the fuel with the air after the consumption valve has closed ( and therefore the burning chamber has 100 % air ) , the maximal end product of the engine can be about 15 % higher than that for gasolene engines.

Therefore, depending on how the fuel is metered, the maximal end product for a H engine can be either 15 % higher or 15 % less than that of gasolene if a stoichiometric air/fuel ratio is used. However, at a stoichiometric air/fuel ratio, the burning temperature is really high and as a consequence it will organize a big sum of N oxides ( NOx ) , which is a criteria pollutant. Since one of the grounds for utilizing H is low exhaust emanations, H engines are non usually designed to run at a stoichiometric air/fuel ratio.

Typically hydrogen engines are designed to utilize approximately twice every bit much air as theoretically required for complete burning. At this air/fuel ratio, the formation of NOx is reduced to near nothing. Unfortunately, this besides reduces the power end product to about half that of a likewise sized gasolene engine. To do up for the power loss, H engines are normally larger than gasolene engines, and/or are equipped with turbochargers or superchargers.

Hydrogen Gas Mixtures:

Hydrogen can be used well in internal burning engines as an linear to a hydrocarbon fuel. Hydrogen is most normally assorted with high force per unit area natural gas for this intent since both gases can be stored in the same armored combat vehicle. If H is blended with other fuels, it normally has to be stored individually and assorted in the gaseous province instantly before ignition. In general, it is impractical to utilize H in concurrence with other fuels that besides require bulky storage systems, such as propane. Gaseous H can non be stored in the same vas as a liquid fuel. Hydrogen ‘s low denseness will do it to stay on top of the liquid and non blend. Furthermore, liquid fuels are stored at comparatively low force per unit areas so that really small H could be added to the vas. Liquid H can non be stored in the same vas as other fuels. Hydrogen ‘s low boiling point will stop dead other fuels ensuing in fuel “ice” . Hydrogen can be used in concurrence with compact liquid fuels such as gasolene, intoxicant or Diesel provided each are stored individually. In these applications, the fuel armored combat vehicles can be formed to suit into fresh infinites on the vehicle. Existing vehicles of this type tend to run utilizing one fuel or the other but non both at the same clip. One advantage of this scheme is that the vehicle can go on to run if H is unavailable.

Hydrogen can non be used straight in a Diesel ( or “compression ignition” ) engine since H ‘s car ignition temperature is excessively high ( this is besides true of natural gas ) . Therefore, diesel engines must be outfitted with spark stoppers or utilize a little sum of Diesel fuel to light the gas ( known as pilot ignition ) . Although pilot ignition techniques have been developed for usage with natural gas, no 1 is presently making this with H.

One commercially available gas mixture known as Hythane contains 20 % H and 80 % natural gas. At this ratio, no alterations are required to a natural gas engine, and surveies have shown that emanations are reduced by more than 20 % . Mixtures of more than 20 % H with natural gas can cut down emanations further but some engine alterations are required. Thin operation of any internal burning engine is advantageous in footings of oxides of nitrogen emanations and fuel economic system.

For hydrocarbon engines, thin operation besides leads to take down emanations of C monoxide and unburned hydrocarbons. As more O is available than required to burn the fuel, the extra O oxidizes more C monoxide into C dioxide, a less harmful emanation. The extra O besides helps to finish the burning, diminishing the sum of unburned hydrocarbons. As with H, the drawback of thin operation with hydrocarbon fuels is a decreased power end product. Thin operation of hydrocarbon engines has extra drawbacks. Thin mixtures are difficult to light, despite the mixture being above the LFL of the fuel. This consequence in dud, which increases unburned hydrocarbon emanations, reduces public presentation and wastes fuel. Another disadvantage is the decreased transition efficiency of 3-way catalytic convertors, ensuing in more harmful emanations.

To some extent, blending H with other hydrocarbon fuels reduces all of these drawbacks. Hydrogen ‘s low ignition energy bound and high firing velocity makes the hydrogen/hydrocarbon mixture easier to light, cut downing dud and thereby bettering emanations, public presentation and fuel economic system. Sing power end product, H augments the mixture ‘s energy denseness at thin mixtures by increasing the hydrogen-to-carbon ratio, and thereby improves torsion at wide-open throttle conditions.

Current Status:

A few car makers have been making some work in the development of hydrogen-powered vehicles ( Ford has late announced that they have developed a “production ready” hydrogen-powered vehicle utilizing an ICE and BMW has completed a universe circuit exposing a twelve or so hydrogen-powered 750i vehicles ) . However, it is non likely that any hydrogen-powered vehicles will be available to the populace until there is an equal re-fuelling substructure and trained technicians to mend and keep these vehicles. Like current gasoline-powered vehicles, the design of each H powered vehicle will most likely vary from maker to maker and theoretical account to pattern.

One theoretical account may be simple in design and operation, for illustration, a thin combustion fuel metering scheme utilizing no emanation control systems such as EGR, catalytic convertor, evaporate fuel case shot, etc. Another theoretical account may be really sophisticated in design and operation, for illustration, utilizing an EGR fuel metering scheme with a catalytic convertor, multiple flicker stoppers, etc. Until such clip that a H substructure exists, hydrogen/natural gas fuel blends provide a logical passage to to the full hydrogen-powered vehicles. These vehicles can run on either fuel, depending on handiness