Hydrogen Asymmetric Rotary TechnologyAddendum to the appraisal of the D.A.R.T. engine. The concepts and technology used for the D.A.R.T. engine are also applicable to many alternative fuels.
Outlined below is a brief appraisal of the Hydrogen Asymmetric Rotary Technology Engine. (H.A.R.T.). The engine as described in the D.A.R.T. appraisal is uniquely suited for the use of hydrogen as a fuel. Its Power density, extremely small size, air scavenge and Atkinson cycle are all the characteristics that are required for a new generation power plant.
It is generally accepted that an ICE burning Hydrogen can never achieve the efficiency of a fuel cell using the same fuel. However an ICE like the H.A.R.T. can extract the maximum potential from hydrogen’s properties and in particular circumstances can outperform fuel cell efficiency. Chart 1. Shows the comparative efficiencies of various ICE’s against that of a fuel cell under a complete load spectrum.
It can be seen that a fuel cell is most efficient at low to moderate loads, and is more efficient than both the petrol and diesel engines. The H.A.R.T. and D.A.R.T. engines produce higher efficiencies than the fuel cell above moderate loads. If these engines are used in hybrid situations operating always at their most efficient load range then they can exceed the overall efficiency of a fuel cell.
In the following chapters we will look at the various properties of hydrogen when used as a fuel in an ICE and relate these to the unique characteristics of the H.A.R.T. engine. Comparisons will also be made to conventional piston engines and Wankle engines.
Hydrogen, when used as a fuel in an ICE has some significant properties over other fuels:
- Wide range of flammability
- Low ignition energy
- Small quenching distance
- High auto-ignition temperature
- High flame speed at stoichiometric ratios
- High diffusivity
- Very low density
- Low emissions
7.1 Wide Range of FlammabilityHydrogen has an extremely wide range of flammability in comparison with all other fuels. As a result, it can be combusted over a correspondingly wide range of fuel-air mixtures. A significant advantage is that a hydrogen ICE can run on a lean mixture.
7.2 Low Ignition EnergyThe amount of energy needed to ignite hydrogen is about one order of magnitude less than that of Petrol. This enables the prompt ignition of lean mixtures and the ease of starting hydrogen engines.
7.3 Small Quenching DistanceHydrogen flames travel closer to the chamber walls before they extinguish. Thus a more complete burning of the fuel-air mixture is assured.
7.4 High Auto-ignition TemperatureThe temperature at which hydrogen fuels auto-ignite is relatively high. This allows the use of high compression ratios and therefore increases the thermal efficiency of the engine. (It is possible to use hydrogen in a diesel configuration but the temperatures and pressures are high).
7.5 High Flame Speed at Stoichiometric RatiosThe flame speed of hydrogen at the chemically optimum air fuel ratio is nearly an order of magnitude higher than that of petrol. This means that hydrogen engines can more closely approach the thermodynamically ideal gas cycle.
7.6 High DiffusivityThe ability for hydrogen to disperse in air is considerably greater than petrol. This facilitates the formation of a uniform mixture.
7.7 Very Low DensityThis is the only real disadvantage that hydrogen has as a fuel. Since hydrogen is a gaseous fuel at ambient conditions it occupies more of the expansion chamber volume than a liquid fuel. The stoichiometric or chemicaly correct A/F ratio for the complete combustion of hydrogen in air is about 34:1 by mass. This is much higher than the 14.7:1 A/F ratio required for petrol. At stoichiometric conditions hydrogen displaces about 30% of the expansion chamber volume, compared to about 1 to 2% for petrol. The advantage of the H.A.R.T engine over conventional piston and Wankel engines is that the H.A.R.T. operates on the Atkinson cycle and not the Otto cycle. This means that the expansion volume is larger than the compression volume and therefor can accommodate this displacement and at the same time extract more energy from the fuel. Diagram 1. Shows the relative volumes of air to fuel for both petrol and hydrogen conventional type engines. Diagram 2. Shows the relative amount of work that can be obtained from both cycles.
The maximum amount of work that can be achieved by Otto cycle engines is indicated by the blue area in diagram 2. The Blue plus the additional red area is the amount of work that can be obtained from the Atkinson cycle engines. This is regardless of the type of fuel used or if the engines are super or turbo charged. Both conventional piston and Wankle engines are constrained by their symmetrical Otto cycle geometries in thus and can never achieve the efficiency of the H.A.R.T. and D.A.R.T. Atkinson cycle engines in this respect.
7.8 Low EmissionsBecause Hydrogen has such a wide range of flammability, Hydrogen engines can run on A/F ratios of anywhere from 34:1 (stoichiometric) to 180:1. It is usual to operate hydrogen engines at very lean mixtures this makes the not only very economical but also very clean. If hydrogen is combusted at close to its stoichiometric ratio then at the high temperatures created some of the nitrogen in the air is converted to NOx. Diagram 3. Shows a typical NOx curve for various A/F ratios.
The red dotted line in diagram 3. Indicates the stoichiometric ratio for hydrogen fuel. Note at an A/F ratio of 70:1 and above the NOx levels fall to almost zero. The downside of this is at high A/F ratios there is also a corresponding fall in power. The H.A.R.T. engine with over twice the power density of a conventional engine is therefore not affected so much by this fall in power. As the H.A.R.T. engine utilises air scavenge this allows the use of EGR systems to be used to their full extent.