GB2558648A

GB2558648A - Diesel internal combustion engine

Info

Publication number: GB2558648A
Application number: GB1700680.0A
Authority: GB
Inventors: Fairney William
Original assignee: Covaxe Ltd
Current assignee: Covaxe Ltd
Priority date: 2017-01-15
Filing date: 2017-01-15
Publication date: 2018-07-18
Also published as: GB201700680D0

Abstract

A diesel internal combustion engine in which compression of the inlet air is completed at top dead centre where fuel injection 22 commences. The fuel is injected into a constant volume chamber when the opposed pistons are held a constant distance apart by cams designed with profiles 18, 19 such that there is a dwell period 21. After fuel injection is completed 26 the combustion chamber remains at constant volume for a period until the cam profiles cause the pistons to move apart again 28. During this phase reduction of oxides of nitrogen and particulate matter takes place at bulk gas temperature, after which the pistons separate and return to bottom dead centre and the gases expand and the temperature reduces accordingly. The fuel injection angle is considerably greater than for a conventional crank-driven internal combustion engine and the peak combustion temperature is thereby reduced resulting in an overall considerable reduction in emissions of oxides of nitrogen and particulates.

Description

(54) Title of the Invention: Diesel internal combustion engine

Abstract Title: Diesel engine with constant volume combustion (57) A diesel internal combustion engine in which compression of the inlet air is completed at top dead centre where fuel injection 22 commences. The fuel is injected into a constant volume chamber when the opposed pistons are held a constant distance apart by cams designed with profiles 18, 19 such that there is a dwell period 21. After fuel injection is completed 26 the combustion chamber remains at constant volume for a period until the cam profiles cause the pistons to move apart again 28. During this phase reduction of oxides of nitrogen and particulate matter takes place at bulk gas temperature, after which the pistons separate and return to bottom dead centre and the gases expand and the temperature reduces accordingly. The fuel injection angle is considerably greater than for a conventional crankdriven internal combustion engine and the peak combustion temperature is thereby reduced resulting in an overall considerable reduction in emissions of oxides of nitrogen and particulates.

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Injection Duration 32, Cam Rotation Degrees

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Diesel Internal Combustion Engine

1. General

In a conventional high-speed gasoline engine the piston moves from Bottom Dead Centre (BDC) where the piston is at the bottom end of the cylinder towards Top Dead Centre (TDC) where the piston is at the top end of the cylinder, thereby compressing a charge of fuel and air vapour. At Top Dead Centre a spark produces a rapid combustion of the mixture and a rapid rise in temperature and pressure ensues. Combustion is completed very rapidly near Top Dead Centre as the charge is already well mixed. Thus with this Constant Volume Combustion (CVC) the full compressive pressure is available to drive the piston down the cylinder bore, giving a good thermal efficiency in what is known as the Otto Cycle.

A conventional high-speed diesel engine has a higher efficiency than a gasoline engine because a higher compression ratio can be used as there is no combustible mixture in the cylinder as the charge air is compressed, so there is no risk of pre-ignition. Such an engine may have a top speed of 4 to 5 thousand revolutions per minute, so there are between 60 and 75 combustion cycle per second. Each complete cycle therefore takes between 13 and 17 milliseconds for a two-stroke engine. In this time the piston moves from Bottom Dead Centre towards Top Dead Centre, thereby compressing a charge of air. As the piston approaches Top Dead Centre fuel starts to be injected into the volume of compressed air. There is a slight delay whilst it evaporates and then it ignites. The pressure in the cylinder rises and as the piston passes through Top Dead Centre and starts to descend, fuel continues to be injected until the piston has descended some way down the cylinder, typically at a crank angle of about 20 degrees after Top Dead Centre. The gaseous products of combustion rise rapidly in temperature and pressure at first but as the piston descends the gases expand and this offsets the rise in temperature and pressure. The net effect is that the combustion takes place at virtually constant pressure. (CPC).

Because the fuel is still being injected whilst the piston is moving down the cylinder, there is a loss of thermal efficiency, depending on how late the fuel injection is cut off. There is therefore an incentive to inject the fuel as quickly as possible at Top Dead Centre. This has lead to a drive to develop very high pressure fuel injection systems requiring sophisticated pulsing techniques to prevent excessive pressure rises leading to an internal explosion known as diesel ‘knock’. Rapid injection at Top Dead Centre also has other major disadvantages. The very high combustion temperatures from burning the fuel so rapidly lead to high production of oxides of nitrogen. Furthermore a by-product of combustion, hydrocarbon particulates, do not have sufficient time to be totally oxidised and so some are emitted with the exhaust gases. The rapid drop in temperature as the gases expand once fuel injection has stopped, results in poor reduction of NOxes and particulates back to their constituent elements.

The net effect of this is that conventional high speed diesel engines have high levels of NOx and particulate emissions which have to be removed by post treatment using expensive catalysts and other chemicals. In some applications this also poses a significant weight penalty. Large low speed diesel engines such as those used in ships do not suffer this problem as the cylinder heads have room for many injectors and there is adequate time to inject all the fuel near Top Dead Centre so these engines achieve almost Otto Cycle efficiency and low pollutant levels. There is therefore a large incentive to develop a high speed diesel engine with inherently low NOx and particulate production.

2. Diesel Engine Combustion Processes

2.1 Constant Pressure Combustion

Figure 1A shows the variation of cylinder gas temperature during the compression stroke from BDC to TDC and from TDC to BDC during the power stroke in a conventional crank-driven diesel engine. Air at or near ambient temperature (1} is admitted to the cylinder whilst the piston is at Bottom Dead Centre, and is compressed to a higher pressure (2} and pressure as the piston moves up the cylinder. As already stated, fuel is injected by injector 23 as the piston nears Top Dead Centre. The fuel sprays out into a pattern of fine droplets and starts to evaporate in the high temperature 2 of the compressed air. However the latent heat of evaporation cools the fuel and and delays ignition. When ignition does take place the rapid release of heat leads to ignition of further fuel and a rapid increase in local temperature by several thousands of degrees Kelvin. This very localised temperature of combustion 5, shown diagrammatically in Figure 6B, leads to a chemical reaction between nitrogen and oxygen in the air and the production of several oxides of nitrogen, NOxes, 24.

These chemical reactions are reversible processes, with NOx formation taking place at the high temperatures, whilst reduction back to the original elements takes place at intermediate temperatures. At exhaust gas and room temperatures the reactions are negligible. Thus as the NOxes diffuse outwards from the combustion zone into the surrounding hot gas, NOx levels start to fall. As fuel injection and combustion ceases the bulk gas temperature 3 becomes virtually uniformly distributed in the cylinder. In a conventional crank-driven engine the gas temperature then rapidly falls to exhaust gas temperature 4 as the piston drops towards BDC. Thus the NOx levels drop only slightly once combustion ceases.

For particulates, the situation is somewhat different. Particles of part-burnt hydrocarbons 25 form just outside the combustion zone and grow by accumulation from the surrounding partly-burnt fuel. Once combustion ceases, the particulates are partly oxidised by the hot gases but the rate of oxidation falls as the piston moves towards BDC and the gases cool. Typical variation of NOx, 6 and particulates 7, with crank angle are shown in Figure 2 for a conventional crank-drive diesel engine.

For an engine where injection ceases typically at a crank angle of about twenty degrees, the injection time is only 800 microseconds. Any attempt to shorten the injection period leads to an increase in peak combustion temperature 5. This leads to a large increase in NOx emissions as NOx production is an exponential function of temperature 5.

2.2 Constant Volume Combustion

In prior art, several diesel engines have been designed to have Constant Volume, or near-Constant Volume Combustion, (CVC). The Scotch Yoke engine (Ref.l) achieves close to the ideal, whilst the opposed-piston cam-drive diesel engine (Fig.3) with non-sinusoidal cam profiles (Ref. 2) can achieve near perfect CVC. This latter engine achieves the cycle by having parallel sections on the two cams which ensure that the pistons remain a constant distance apart during fuel injection and combustion. With this cycle near Otto Cycle thermal efficiency can be achieved.

The Present Invention - Low-Polluting Diesel Engine

In the present invention a high-performance diesel engine with low pollution characteristics is achieved by using an opposed-piston two-stroke or four-stroke cam-drive engine with cam profiles and fuel injection timing phased to ensure a longer fuel burn time at lower peak combustion temperature than with a conventional crank-driven engine.

The features of the invention are shown diagrammatically with reference to Figures 3, 4 and 5.

In Figure 3, 8 is a cylinder containing two opposed pistons 9 and 10, which engage via rollers 11 and 12 or other means with opposed cams 13 and 14 mounted on a drive shaft 15. 16 is a fuel injector operated by a mechanical, electronic or hydraulic controller 17 (Not shown)..

In Figure 4A, 18 and 19 are the profiles of the two cams 13 and 14 showing the piston displacement from Top Dead Centre for one cycle of the motion of the pistons 9 and 10. The distance 20 between the pistons 9 and 10 determines the volume of gas between the said pistons.

is the angle of cam rotation at which the distance 20 between the said cams becomes constant and 28 is the angle at which the said cams start to move apart. For the purposes of clarity the angle 27 is taken to be the Top Dead Centre position and is arbitrarily set to zero degrees.

In Figure 4B, 21 is the angle of cam rotation over which the distance 20 is constant and angle 22 of the cam rotation, is the angle at which fuel injector 16 starts injection and 24 is the angle of cam rotation at which combustion of the fuel 25 commences. 26 is the angle of cam rotation at which fuel injection ends.

Figure 5 shows the temperature profiles of the combustion gases in A, a conventional crank-drive diesel engine, and B, a diesel engine of the present invention. In both types of engines the inlet air is compressed from Bottom Dead Centre to Top Dead Centre in section 23 of the complete combustion cycle 29.

In the case of the Conventional crank-drive engine Figure 5A, where the mean cylinder gas temperature is plotted against the angle of crankshaft rotation 36, fuel injection 22 starts typically at or near Top Dead Centre 27. For the purposes of this argument fuel injection 22 is taken to be at Top Dead Centre 27 and to last an angle 32 typically twenty degrees of crank angle rotation θρ until it is cut off at crank angle 26. During this period the piston in the cylinder descends towards Bottom Dead Centre and so the combustion takes place at approximately constant pressure. The hatched area 30 represents the fixed amount of fuel Qp injected into the cylinder at constant pressure. As fuel injection 30 commences thr fuel spontaneously ignites and a very localised temperature 5 arises which combusts the fuel and also converts some of the oxygen and nitrogen in the surrounding air to oxides of nitrogen, NOxes. This temperature Tp is typically of the order of 2,500 to 3,000 degrees Kelvin.

In the present invention the Inventive Step is to move away from the conventional crank-driven engine with Constant Pressure combustion in which the emphasis is on very rapid fuel injection to a cam-driven engine in which combustion takes place at Constant Volume with fuel injection taking place over a longer period typically but not necessarily of the order of 50 to 60 degrees of cam rotation angle.. This change results in combustion taking place at a lower temperature resulting in a reduction in the production of oxides of nitrogen and particulate emissions.

The compression, combustion and expansion cycle of the present invention is shown in Figure 5B where the mean cylinder gas temperature is plotted against the angle of cam rotation 37. Compression of the inlet air is completed at Top Dead Centre where fuel injection 22 commences. In this case the fuel is injected into a constant volume chamber because the opposed pistons 9 and 10 are held a constant distance apart for a duration 31 of cam rotation angle θν. Fuel injection 22 lasts for a cam rotation angle 32 and the hatched area 33 is the quantity Qv of fuel injected. After fuel injection is completed at 32 the combustion chamber remains at Constant Volume until cam rotation angle 31 is reached. During this phase some reduction of oxides of nitrogen takes place at bulk gas temperature, after which the pistons 9 and 10 separate and return to Bottom Dead Centre and the gases expand and the temperature reduces accordingly.

For the evaluation of the present invention two published papers (References 3 and 4) concerning emissions from diesel engines have been consulted.

If the fuel injected at Constant Volume 30 as shown diagrammatically in Figure 5B is the same quantity as that injected at Constant Pressure 33 as shown in Figure 5A, it can be demonstrated that the peak combustion temperature T_v for the constant volume case is related to that for the Constant Pressure Tp case by the equation

Where Tf is the activation temperature for the hydrocarbon fuel oxidation and Ln signifies the Napier logarithm.

3.1 NOx Emissions

In the present invention because NOx production is an exponential function of peak combustion temperature the consequent reduction in this temperature is very significant and it can be further demonstrated that the NOx level produced for the same fuel burn at Constant Volume N_v compared to that produced at Constant Pressure Np can be approximated by the equation:4

N_v/Np - θ_ρ/θ_ν

In the present invention the Constant Volume cycle has a further advantage over the Constant Pressure cycle because the thermal efficiency is inherently higher for a given compression ratio. Typical values for a compression ratio of 20 are 46% for the CPC cycle and 61% for the CVC cycle. Taking these figures into account Figure 6 shows the variation in NOx emissions 34 with fuel injection duration 32 compared to that of a CPC cycle with injection duration 32 of twenty degrees of crank shaft rotation.

3.2 Particulate Emissions

The production and oxidation of particulates in a diesel internal combustion engine is more complex than the production of oxides of nitrogen as there are more processes taking place.

In the present invention particulates 25 form from free radicals released during the combustion phase 33 whilst fuel is injected. At the same time particulates diffuse into the bulk gas zone 3 and are removed through oxidation. Once combustion ceases oxidation continues in the high bulk gas temperature 3 which is maintained during the remainder of the constant volume phase 31. Because of the length of this period, typically but not necessarily of the order of fifty to sixty degrees of cam rotation angle 37 more elimination of particulates takes place than in the cylinder of a conventional crank-drive diesel engine.

In the present invention a similar analysis for particulate production as for NOx production is possible. The results for particulate production 35 are also shown in Figure 6 where it is seen that the reduction is even greater than for NOx emissions.

References

1. P. R. Raffaele, and M. J. Raffaele. ‘Scotch Yoke Engine’. US Patent 7210397 B2, May 1^st 2007

2. William Fairney, FairDiesel Limited. ‘Diesel Internal Combustion Engine’. UK Patent GB2453131, 19^th September 2012.

3. Jean Boulanger, Fenshan Liu, W. Stuart Neill, and Gregory J. Smallwood. ‘An Improved Soot Formation Model for 3D Diesel Engine Simulations’. Journal of Engineering for Gas Turbines and Power, Vol. 129, July 2007.

4. Chrys Correa, ‘Combustion Simulations in Diesel Engines using Reduced Reaction Mechanisms’, Dissertation to the Rupertus Carola Unuversity of Heidelberg, June 2000.

Claims

1) A diesel internal combustion engine in which fuel injection takes place into a cylinder wherein the pistons have a motion such that the combustion takes place at constantvolume over such a fraction of the complete compression and combustion and expansion cycle that the fuel is burnt at reduced temperature thereby produces considerably reduced levels of oxides of nitrogen to those experienced in a crankshaft or other type internal combustion engine having quasi-sinusoidal piston motion.

2) A diesel internal combustion engine in wherein the piston has a motion such that the combustion products remain in a cylinder at constant-volume over such a fraction of the complete compression and combustion and expansion cycle thereby maintaining a higher temperature after combustion has ceased and producing considerably reduced levels of hydrocarbon particulate matter to those experienced in a crankshaft or other type internal combustion engine having quasi-sinusoidal piston motion.

3) A diesel internal combustion engine having two or more cylinders in accordance with Claims 1) and 2).

Amendment to Claims have been filed as follows

1. A cylinder for a diesel internal combustion engine, containing two pistons each driving a cam on a drive shaft and having a fuel injector arranged to inject fuel

5 into the cylinder, in which the cams are shaped so that a constant volume is defined in the cylinder between the pistons over a constant volume fraction of a complete compression, combustion and expansion cycle of 360 degrees of the engine as the drive shaft rotates, and in which the fuel injector is arranged to inject fuel into the cylinder during the fraction of the cycle of the engine over a fuel injection fraction of

10 the cycle of the engine which extends to between 50 and 60 degrees of the cycle.

2. The cylinder of claim 1, in which the constant volume fraction is larger than, but contains, the fuel injection fraction of the cycle.

15 3. The cylinder of any preceding claim, in which the constant volume fraction of the cycle commences at the same time as the fuel injection fraction.

4. A diesel internal combustion engine comprising a cylinder in accordance with any preceding claim.

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Application No: GB1700680.0 Examiner: Nicholas Wigley