FR2765238A1 - NEW LIQUEFIED GAS FUELS FOR INTERNAL COMBUSTION ENGINES - Google Patents

Abstract

The invention concerns novel fuels consisting of a liquefied mixture of at least two gases. The invention is applicable to diesel engines with controlled ignition.

Description

 The present invention relates generally to a new fuel for internal combustion engines, and more particularly to a new fuel reducing the polluting emissions of internal combustion engines.

 At present, the most efficient engines in terms of fuel consumption are direct injection diesel engines and gasoline engines, four or two stroke, operating in lean mixture. The diesel engine is particularly interesting in terms of fuel consumption, because of its better thermodynamic efficiency than that of gasoline engines.

 In practice, the efficiency of an internal combustion engine increases with its volumetric ratio, this volumetric ratio being from 16 to 23 for diesel engines and from 9 to 11 for petrol engines. It is the sensitivity of gasoline engines to the rattling phenomenon - abnormal combustion which can lead to the destruction of the engine - which confines these gasoline engines to these relatively low volumetric ratios.

 Unfortunately, diesel engines like petrol engines running on lean blends do not have effective aftertreatment, particularly with regard to particles and nitrogen oxides.

 On two-stroke spark ignition engines, it is possible to use ATAC type combustion which improves engine efficiency and therefore its fuel consumption while reducing polluting emissions. This type of combustion has only been observed on two-stroke engines and over a reduced operating range.

This limitation of the operating range in ATAC combustion combined with the fact that the catalytic elimination of nitrogen oxides is not yet satisfactory, has confined this type of combustion to the domain of the laboratory.

 At the moment, catalysis is the key point to reduce polluting emissions from the exhaust of petrol and diesel engines.

In the case of diesel engines, it is possible to further reduce polluting emissions by adding particulate filters and injecting urea to regenerate them.

 Another solution, to minimize polluting emissions, consists of using so-called alternative fuels to conventional fuels such as petrol and diesel such as dimethyl ether (DME) for diesel engines or liquid petroleum gas (LPG) or natural gas (NG) for positive-ignition engines.

 Unfortunately, the energy consumption obtained by using alternative fuels (LPG and NG) is no better than with conventional fuels.

 In addition, these alternative fuels all have drawbacks.

 Dimethylether is a liquefiable gas under medium pressure (5 bars at 20 "C), known for its combustion which does not generate smoke and has a low rate of emission of nitrogen oxides. We therefore considered using it as fuel However, dimethyl ether is a highly compressible product whose characteristics change rapidly with temperature. As a result, its use as fuel necessitates difficult injection development. It has a low energy density (19 MJ / 1) compared to diesel (35 MJ / 1), which will require doubling the capacity of the tank compared to its diesel equivalent, for the same operating autonomy of the engine. It is extremely corrosive to elastomers and polymers, which requires the use of specific materials. Finally, dimethyl ether has a very low viscosity. For all these reasons, it would be necessary to significantly modify the system. injection of a diesel engine to make its operation compatible with the use of DME as fuel.

 Natural gas also poses problems with regard to its storage in compressed form because it has a low volume PCI and in liquefied form (at approximately - 163 "C) the storage tanks necessarily cryogenic, are expensive. Natural gas supply systems are more expensive than LPG supply systems and the catalysis of unburnt hydrocarbons in natural gas (mainly methane) is more difficult than that of petroleum gasoline or LPG.

 Liquefied petroleum gas (LPG), although very favorable from the point of view of pollution, is comparable to petrol in terms of energy consumption.

 To reduce the polluting emissions of diesel engines, we have also considered a "dual fuel" operation. This operation uses two fuels, one as a "match" with good diesel behavior (cetane number), the other as a charge for its combustion characteristics. However, this operation involves the use of two fuel supply systems and two tanks. In addition, the admission of one of the fuels in a homogeneous mixture to the intake generates flame jams and adsorption of hydrocarbons on the walls of the cylinders. This results in an increase in the unburnt hydrocarbons emitted from the exhaust.

 It has also been proposed to convert diesel engines operating on diesel to spark ignition engines operating on natural gas monofuel. In this case, the compression compression ratio is reduced to around 12 to 14 to 1 (instead of 16 to 19), the injector is replaced by a spark plug and the diesel injection system by a supply of natural gas. Natural gas is then used in a stoichiometric mixture with a three-way catalyst or in a lean mixture with an oxidation catalyst.

 However, this solution requires significant modifications both on the engine and on the vehicle. Natural gas is generally stored on board the vehicle in compressed form at 200 bar, which requires heavy and bulky tanks.

 Finally, the efficiency of the ignition cycle is lower than that of the diesel cycle.

 There is therefore a need for a single fuel overcoming the above drawbacks.

 According to the invention, a new fuel has been developed which overcomes the drawbacks of alternative fuels of the prior art.

 The new alternative fuel for an internal combustion engine according to the invention consists of a liquefied mixture of at least two gases. Preferably, one of the gases of the liquefied mixture is self-igniting by compression.

 The new alternative fuel according to the invention is still preferably constituted by a liquefied mixture of petroleum gas (LPG) and dimethyl ether (DME), a liquefied mixture of petroleum gas (LPG), dimethyl ether (DME) and natural gas (GN) or a liquefied mixture of dimethyl ether (DME) and natural gas (GN).

 Dimethylether is soluble in liquid hydrocarbons, in particular LPG, while methane, the main constituent of natural gas, is soluble in liquid hydrocarbons, in particular LPG and DME at pressures at which these two fuels are liquid. The proportion of the various constituents of the mixture of the new alternative fuel according to the invention depends on the intended use - spark ignition or diesel engine, light or heavy vehicle - and can be easily determined by means of routine tests.

 The new alternative fuel according to the invention requires only a single tank which, as in the case of LPG, is a medium pressure tank (30 bars) and a single fuel injection system.

 It is possible to use the new fuel as the sole fuel both in spark-ignition engines and in diesel engines, by varying the proportions of each component. Combustion of the “dual fuel” or “ATAC” type is therefore possible.

 Used as fuel for a diesel engine, the new fuel according to the invention does not require significant modifications to the engine and makes it possible to maintain the performance thereof, or even to improve it. It is perfectly compatible with direct injection technologies, because it therefore does not cause flame jams.

The DME thanks to its excellent cetane index, ignites by compression in multiple places of the combustion chamber thus providing multiple ignition points to the rest of the charge consisting of LPG, NG or a mixture of the two . In addition, the new fuel has a high energy density due to the presence of LPG and / or NG. Thus, a fuel comprising LPG and 10% of
DME has a volume energy of 24.5 MJ / 1.

 The use of this new fuel almost completely eliminates the emission of particles, because the hydrocarbons of this fuel are limited to hydrocarbons containing at most 4 carbon atoms. It greatly reduces the production of nitrogen oxides due to the presence of oxygen (DME) and the smoother combustion inherent in gaseous fuels (especially with a diesel engine). CO2 emissions are also reduced by around 10%.

 Finally, as it is possible to optimize combustion for engine emissions that are very low in nitrogen oxides (absence of particles), the exhaust after-treatment system can be reduced to the oxidation catalyst alone.

When used with a positive-ignition engine, it has the following advantages
it is not necessary to have a direct injection into the engine (but there is no contraindication in this). The cost of adapting the fuel supply is of the same order of magnitude as for LPG and current engines are suitable for this application.

 stoichiometric operation is possible and 3-way catalysis very effective in treating polluting emissions.

 this fuel has a particularly interesting lean mixture application potential. I1 provides a gain in efficiency compared to the lean mixture operation of the gasoline because of its rapidity of combustion. Furthermore, the improvement in combustion stability would result in a lowering of the lower limit richness of stable operation and therefore a reduction in engine output emissions of nitrogen oxides and unburnt hydrocarbons as well as in consumption.

it makes it possible to operate with a combustion which can start with a combustion of the controlled ignition type and which continues with a combustion of the "dual fuel" type and / or which is from the start a combustion akin to an ATAC combustion, it that is to say, which is self-sustaining, which has the consequence of reducing pollution and increasing yields. In this case, the
DME plays the role of residual gas from the 2-stroke combustion engine
ATAC. The advantage is that, unlike ATAC combustion, it would be possible to obtain this phenomenon of self-sustained combustion over a greater range of load and speed; and that 2-stroke engines would not be the only ones to benefit from this type of combustion.

 Refueling according to the invention can be carried out by means of a refueling station allowing an independent metering of the fuel constituents. This station includes a separate tank for each of the fuel components. Each tank is connected to a separate metering device, the flow rate of which is adjustable by the operator independently of the other metering devices. These metering devices are themselves connected to a mixer fitted with a refueling hose extended by a sealed LPG type connection. A flow meter reading of the flow rate of each component makes it possible to ensure the accuracy of the metering.

 As is conventional, the storage tanks can be storage tanks on activated carbon at a pressure of 30 bars or tanks at a pressure of 200 bars, but in the latter case, a compressor is necessary.

 This system, depending on the adjustment made by the operator on each metering device, makes it possible to continuously deliver either one of the constituents or a mixture of the constituents in the desired proportions.

 Thus, a single refueling system makes it possible to refuel both diesel and spark-ignition vehicles.

 Only one type of pump would therefore be necessary in service stations for the delivery of liquefied gaseous fuels.

Claims (7)

 1. New alternative fuel for internal combustion engine, characterized in that it consists of a liquefied mixture of at least two gases.  2. New fuel according to claim 1, characterized in that it is suitable for the operation of a spark-ignition engine.  3. New fuel according to claim 1, characterized in that it is suitable for compression ignition type operation.  4. New fuel according to claim 1, characterized in that one of the gases of the liquefied mixture is self-igniting by compression.  5. New fuel according to any one of claims 1 to 4, characterized in that it is constituted by a liquefied mixture of petroleum gas and dimethyl ether, a liquefied mixture of propane gas, dimethyl ether and natural gas or a liquefied mixture of dimethyl ether and natural gas.  6. New fuel according to claim 1, characterized in that one of the gases of the liquefied mixture has a high cetane number.  7. system for refueling according to any one of claims 1 to 6, comprising a separate storage tank for each of the fuel constituents connected to a separate metering device whose flow rate is adjustable independently of the other metering devices, these metering devices being connected to a supply hose.