#1 How does it work?
A conventional internal combustion engine which does not consume atmospheric air, thus stores oxygen onboard (for mobile applications) and recycles a working fluid, a non-nitrogen inert gas. The exhaust of a hydrogen engine is nothing but water vapor, no carbon dioxide is produced, therefore there is no need for exhaust, as there is no pressure buildup from carbon dioxide gas being released with the combustion of hydrocarbons. Steam is the only bi-product. Steam is condensed in a specially designed condenser. Hydrogen engines inherently want to operate on a closed cycle, thus, any hydrogen engine designer not running on a closed-cycle is failing to realize a potential 50% increase in efficiency, and not fully exploiting hydrogen’s unique properties to eliminate all pollutants. A closed cycle hydrogen engine by definition will have 100% combustion efficiency, since no unburned hydrogen is lost through the exhaust.
#2 What are the major advantages over conventional atmospheric engines?
The first major advantage is the elimination of the only pollutant produced from hydrogen combustion: NOx, finally making hydrogen engines fully clean, as they currently are “quasi” zero-emission, producing no carbon dioxide or monoxide, as hydrogen contains no carbon, but still producing high amounts of nitrogen oxides, especially at stoichiometric mixtures.
The second major advantage is the ability to use pure hydrogen in a diesel cycle. Currently, hydrogen cannot be used in a diesel engine due to the very high auto-ignition temperature, which necessitates a 35:1 compression or 200 °C intake air temperature to achieve sufficient compression temperature for auto-ignition. Neither of these options is feasible. Pilot fuel can be used, but must be a hydrocarbon fuel since a high cetane number is required. The reason we want to use a diesel cycle is due to the great efficiency advantage over the Otto cycle, decreasing operating costs.
A Hydrogen direct injection compression ignition engines with heated intake air (non-closed cycle nitrogen-oxygen) was demonstrated to achieve a tremendous 53% efficiency advantage over diesel fuel using a 10 hp test engine. The diesel engine was 27.9% efficient, vs 42.8% for the same engine with hydrogen injection. The power output of the hydrogen injection version was 15% higher. A hydrogen fuel injector was custom built in place of a swirl-chamber diesel injector. (JMG Antunes, R. Mikalsen, A.P. Roskilly, Newcastle University, International journal of hydrogen energy, 2009).
The huge increase in efficiency is mainly a function of the much higher cylinder pressure rise due to hydrogen’s higher flame speed. Thanks to the faster cylinder pressure rise, more work is accomplished in a much shorter period of time, this results in less heat lost by conduction through the cylinder wall, since there is less time for conduction to occur. Heat loss through the cylinder wall into the coolant was estimated to be 42% of the total energy input for the diesel version, but only 17% for the hydrogen direct injection version. (JMG Antunes, R. Mikalsen, A.P. Roskilly, Newcastle University, International journal of hydrogen energy, 2009).
This data illustrates that hydrogen is a far superior combustion engine fuel, despite “experts” claiming only fuel cells can be used with hydrogen, and that combustion engines are not appropriate. No other fuel available can increase the efficiency of a diesel engine by 53% other than hydrogen.
The third major advantage is an across the board increase in efficiency of 25% regardless of whether a spark ignited or diesel engine is used. (P.C.T De Boer, J F. Hulet, International Journal of Hydrogen Energy, 1980). The reason for the increase in efficiency is due to the higher heat of the non-nitrogen gas under the same compression, resulting in higher combustion temperature and greater power produced. With this increase in efficiency, a spark ignited engine can be 45% efficient, vs 35% for conventional Otto cycles. A diesel engine version could be 55%, making hydrogen engines equally efficient as fuel cells, yet at a fraction of the cost.
#3 Why can our engine operate on a diesel cycle whereas other hydrogen engines such as Keyou and BeHydro cannot?
Thanks to the closed-cycle, we can use an inert gas (not nitrogen and air) which heats up to a much greater degree under the same compression (called the specific heat ratio) allowing us to attain a temperature sufficient to auto-ignite hydrogen, without requiring a high cetane number hydrocarbon pilot fuel, Dimethyl Ether, Biodiesel B100, or straight diesel fuel can be used, but not in a closed-cycle, as CO2 buildup will be unmanageable. With Argon gas, with an intake temperature of 100 °C, a 17:1 compression ratio results in a temperature of 900 °C, 300 °C over the auto-ignition temperature. The higher intake temperature is a result of the closed cycle system, which routes the exhaust right back into the intake, instead of cooler ambient intake air in an open-cycle. With an intake air temperature of 70 °C, a compression ratio of 35:1 is required to auto-ignite hydrogen with an oxygen-nitrogen mixture. Making hydrogen compression hydrogen engines nearly impossible, limiting hydrogen engines to Otto cycles, which are limited to 35% efficiency.
#4 How is our engine different from existing internal combustion engines?
It is not different at all. The engine can either be spark ignited, or compression ignited, exactly the way current engines reciprocating engines operate. In compression-ignition versions, the fuel (hydrogen) is injected directly like conventional diesel engines, oxygen is mixed with the recycled intake argon at the stoichiometric ratio. In spark-ignited versions, the hydrogen is injected in the intake manifold in the same fashion natural gas engines operate. The engine type can be 4-stroke, 2 stroke, a gas turbine or Wankel. Just about any combustion engine can operate on our closed-cycle. Any combustion cycle, including Brayton, Otto, Diesel, Atkinson, and Rankine can be used.
#5 Since it’s a closed cycle and does not consume atmospheric air, we have to consume oxygen, does this increase the cost? Short answer: On the contrary, it significantly reduces operating costs thanks to higher efficiency.
Oxygen both in high pressure gaseous (890 kg/m3 at 700 bar, 1140 kg/m3 liquid) and liquid state is very dense. 1kg of hydrogen requires 7.4 kg of oxygen for complete combustion, what we call the stoichiometric ratio. For atmospheric air, this ratio is 2.9% hydrogen to atmospheric air by mass. This small ratio is what makes the closed-cycle so attractive, very little oxygen is actually required compared to the engines mass airflow in conventional operation, this is because diesel engines run very lean, low equivalence ratio, to minimum NOx formation. Thus if our engines consumed 11 kg of hydrogen per hour or per 60 miles (based on 5.9 mpg for a typical semi-truck) we only need 80 kg of oxygen. Storing enough oxygen to drive 850 km only requires 580 liters for liquid storage.
The weight of the oxygen storage is also minimal for mobility use. For a hypothetical 9000 lb light-duty truck (Ford F-450) with a 500 miles range, the oxygen tank weighs 240 lbs, plus 440 lbs for the oxygen, representing only 4.5% of the vehicle’s GVWR. Thus, storing liquid oxygen onboard a vehicle, may at first seem almost science-fiction-like, but in reality, is it least of the operators’ worries.
For stationary applications, oxygen can be stored in large spherical liquid tanks or can be piped directly in low-pressure gaseous form. For stationary storage, due to oxygen’s very high density, the “boil-off rate” is quite small. The liquefaction of oxygen is very easy, due to oxygen having an inversion temperature above ambient, requiring no pre-cooling the way hydrogen requires. Liqiufiyng oxygen for transportation and storage is not only cost effective, but technically simple, requiring only compressors and expanders, all of which are widely available on Alibaba.com
The cost of oxygen, either liquid or gaseous is very low due to its abundance. Liquid oxygen costs only $0.05/kg in India (Metals and Minerals Trading Corporation of India, 2016) and $0.09/kg in the U.S. (Matheson Tri-Gas, 2017) Resulting in hourly operating costs of $4 and $7.3 respectively for a 300 hp engine.
A hypothetical 1000 hp (750 kw) diesel generator with a BSFC of 0.33 lbs/bhp-hr (41% efficiency) would consume 47.6 gallons of diesel fuel per hour, with the efficiency improvements from the closed cycle, this would be reduced to 40 kg of H2 per hour, requiring 254 kg of oxygen, at a cost of $14.8 per hour, contrasted to $140/hr for $3.5/kg clean hydrogen from electrolysis. Netting an electricity cost of $0.020/kwh. In comparison, a spark-ignited non closed cycle engine with a BSFC of 0.4 lbs/bhp-hr (34% efficiency) would consume 66 kg of H2 per hour, costing $231/hr, netting an electricity price of $0.31/kwh. This clearly illustrates that for any application, mobility or stationary power generation, the closed-cycle is much more cost effective and would yield operators much greater profits. A fuel cell powerplant, averaging over $2000-5000/kw, is not even comparable, as the acquisition cost for a 750 kw unit would be astronomical, totaling over a million and half dollars.
#6 What are the necessary modifications required to convert an engine to closed-cycle?
That depends on whether a spark-ignited or diesel engine is used. For spark ignition, the first modification required is a compression ratio reduction to 5.5:1 for a typical 11:1 compression ratio gasoline or natural gas engine. Reducing the compression ratio simply involves raising the cylinder head a small distance, this can be done by installing a thicker metal cylinder head gasket. The increase in height is around 0.2”. The fuel system on the conventional spark-ignited engine is removed and replaced with low-pressure constant flow hydrogen injection in the intake manifold. No specialized injectors are needed, simply a constant flow of low-pressure hydrogen is fed into the intake manifold.
Regardless of whether diesel or spark-ignited, proper crankcase ventilation is required for any hydrogen engine to ensure no build-up of hydrogen in the crankcase occurs. This can be accomplished by inserting a small vent in the crankcase, equipped with a hydrogen sensor, to monitor the potential accumulation of concentration of unburned gas.
The second modifications after the fuel system is the exhaust system. The exhaust tubing, condenser, and dual argon-CO2 vent tanks are filled up with argon, which is continuously recycled. Two tanks are used in order to maintain a 15% CO2 to argon ratio, as a small amount of lube-oil is burned, releasing CO2, the dual tanks are continuously switches, when one tank reaches 15% CO2 concentration, it is emptied and refilled with pure argon. More on this can be found on question #9.
The water vapor condenser is installed just before the intake, so that the exhaust gases have had sufficient time to cool down to the 100 °C intake temperature, allowing a certain percentage of the water vapor to have condensed. For vehicle use, The liquid oxygen is used to acceleation the condensing of the water vapor, for stationary applications, where space is not as constrained, a larger condenser is used, eliminating the need for cooling.
The third modification required is the installation of an oxygen regulator. The oxygen regulator is located on the oxygen intake hose, which can be a low-pressure flexible line, connected to a high-pressure storage vessel for vehicle use, or a low-pressure supply line for stationary applications. The oxygen regulator is calibrated to provide a 7.4:1 ratio of oxygen to hydrogen by mass at any given engine speed. The regulator can be electronic or mechanical.
The fourth modification is the installation of two hydrogen sensors. A hydrogen sensor is installed in the crankcase, to warn the operator if any gaseous hydrogen has accumulated in the crankcase due to a blockage in the venting system.
The second hydrogen sensor is installed in the exhaust line, to measure combustion efficiency. If a high concentration of unburned hydrogen is observed, the oxygen regulator could be malfunctioning and the engine could be running too rich, the oxygen regulator is recalibrated to increase oxygen flow, until the unburned hydrogen concentration is reduced to normal levels.
For a diesel cycle, the modifications required are somewhat more elaborate but isolated to the injection system. In a diesel engine, precise injection of the fuel is critical for smooth and reliable operation. Intake injection is possible, through what is called homogeneous charge compression ignition (HCCI). Fuel is mixed with the intake air, in this case hydrogen, oxygen and argon, then compressed until auto-ignition. This cycle is highly efficient, one study shows up to 12% more efficient than direct injection. (JMG Antunes, R. Mikalsen, A.P. Roskilly, Newcastle University, International journal of hydrogen energy, 2008). The main issue with HCCI is timing difficulty, which results in rough and sporadic operation. Currently, no HCCI engine is in operation. Pochari Hydrogen strongly believes direct injection is the best option.
To perform the closed-cycle conversion for diesel, the first step is to remove the fuel injection system. The unit injectors, injection pump, fuel lines and ECM are removed. Newly built hydrogen injectors designed to fit in the same slot are installed. A new ECM is designed for injection timing, and in some cases, a mechanical timing system could be used. Modern diesel injection systems are designed primarily to minimize particulate emissions. Multiple injection cycles per stroke are common, very high pressure is used to atomize the fuel as much as possible to ensure complete combustion. All of this requires precise electronic timing. A large portion of the modern electronic diesel injection system is solely designed for emission requirements, not for performance or efficiency. With a closed cycle hydrogen engines, all current efforts to minimize emissions, often at the expense of performance, can be completely eliminated, freeing the designer to focus solely on performance and efficiency.
Two types of hydrogen injectors are available.
A “common rail” version where the hydrogen line is pressurized to the final injection pressure, the injector essentially serves only as a valve, to spray a certain amount of already high pressure gas in the chamber. This type of injector does not incorporate a plunger used to pressurize the fuel just prior to injection. Hydrogen injection pressurization is required due to the very high cylinder pressure found in diesel engines, up to 2000 psi, thus 3000 psi is required for a sufficient margin.
This type is most attractive where high-pressure hydrogen gas is available, such as a vehicle which uses high-pressure or liquid tanks.
For stationary engines, which may run off low-pressure gas, a different type of injection system can be used. This system works whereby the low-pressure gas is fed to the injector, and a plunger pressurizes the gas to around 3000 psi, 1000 psi above peak cylinder pressure. This type of injector is more complicated, but is more attractive for stationary engines that that run off low-pressure hydrogen as it eliminates the requirement of having a micro hydrogen compressor on the engine.
If the first injection system is used where high-pressure gas is unavailable, an on-board compressor is required to pressurize the hydrogen to 3000 psi, serving essentially as a gaseous injection pump.
Regardless of the type of injector chosen, the injectors need to be custom made for the particular engine converted, as no manufacturer currently offers hydrogen injectors for sale. The existing injector profile area can be copied to design the new injector, so the newly built hydrogen injector fits in the existing slot and hole in the cylinder head, eliminating the need to make modifications to the cylinder head, which is inadvisable.
Three types of injectors are available
#1 Piezoelectric crystal actuation
#2 Electromagnetic solenoid actuation
#3 Hydraulic, mechanical or electronic actuation.
The hydrogen injectors do not greatly differ from a conventional liquid fuel injector. No fuel atomization is required, eliminating the need for extremely high operating pressures commonly found in modern diesel engines, this reduces the design requirements on the injector.
A few challenges arise with hydrogen injection.
The first issue is embrittlement of many metals caused by contact with high-pressure gaseous hydrogen. Stainless steel, carbon steel and titanium are susceptible to embrittlement among other, aluminum alloys appear to be quite resistant to embrittlement. To minimize embrittlement, the inside of the injector can be coated with a liner made of high-density ceramic or epoxy, eliminating contact between the gas and the metal.
The second potential issue is the lack of lubricity with a gaseous fuel, but this has not proven to be an issue as many natural gas injectors have been successfully implemented and have proven to be reliable.
In summary, hydrogen injectors are not a major technical challenge compared to liquid fuel injectors, and do not differ greatly from existing natural gas injectors, with the exception of requiring special attention to embrittlement.
Pochari Hydrogen is currently designing a hydraulic, mechanically actuated unit injector. This injector is based on proven Caterpillar hydraulic electronic unit injector (HEUI) technology.
#7 Why use this cycle over conventional spark-ignited hydrogen engines?
Previous experiments with spark-ignited hydrogen engines have not shown promising results for one single reason. Hydrogen is very attractive as a means to reduce emissions, but in the absence of a closed-cycle, high amounts of nitrogen oxides are formed due to hydrogen’s high combustion temperature. Nitrous Oxide, one of the seven oxides of nitrogen, is an extremely potent GHS.
Nitrogen oxidization is a function of combustion temperature and dwell time, essentially how long the combustion occurs. The formation of NOx increase dramatically above 2500 F, peak flame temperature in diesel combustion can reach 4000 F, resulting in very high NOx formation, up to 1000 PPM. Thus any attempt to design a dual-fuel hydrogen-diesel engine such as BeHydro will be a failure. In the case of hydrogen, dwell time is reduced due to much higher flame speed (7-8 higher laminar flame speed). In the case of combustion temperature, hydrogen tends to burn hotter than liquid hydrocarbon fuels, which results in more NOx formation. As a result, hydrogen engines which run at a stoichiometric ratio, 2.9% hydrogen to air by mass, emit excessively high NOx emissions, defeating the purpose of using hydrogen in the first place. As a result, hydrogen engine designers typically run the engine at a 0.5 Equievelant ratio, or twice the amount of air than stoichiometric. This leads to a drastic reduction in power output, around 50% less power is developed. An example of this is the MAN hydrogen engine developed for buses in 2006. The engine was a spark-ignited,12:1 compression ratio, direct injection, but only produced 270 hp at 2200 rpm with 13 liters. The port injection version of this engine only developed 200 hp! This is about half the power produced by modern diesel engines of similar displacement. As a result, a customer that requires a 500 hp engine, be it for a truck or generator, would need 26 liters, doubling the weight and volume of the engine and doubling the acquisition and maintenance cost. The other obvious reason aside from the reduction in power density is the fact that currently, hydrogen cannot be used in diesel engines, limiting hydrogen to 35% efficient Otto cycle engines. In summary, conventional spark-ignited lean-burn hydrogen engines are very unattractive compared to conventional hydrocarbon engines, and completely uncompetitive and obsolete when a closed-cycle is available.
#8 Why hasn’t it been done before?
Mainly because all the research and interested is focused on batteries, and if there is any interest in hydrogen, it’s almost invariably focused on fuel cells. As a result, the easiest and simplest solution was ignored. Apart from us, no one aside from Keyou Gmbh and more recently a newly formed company BeHydro, is focusing on hydrogen engines, and no one in the world yet is working on closed-cycle direct injection compression ignition hydrogen engines. We encourage engineers, designers and inventors to build this technology as soon as possible, so we have made it open source.
The main applications for the closed-cycle is for mobility, mainly cars, light-duty trucks, heavy-duty trucks, marine, freight trains and also for non-mobilty, mainly decentralized power generation.
#9 Is lube-oil combustion and subsequent CO2 buildup in the closed-cycle going to be an issue?
Not at all. Modern diesel engines consume between 0.2-0.8 grams of lube oil per Kwh. This means if we have a 100 kw engine or 134 hp, we consume 0.176 lbs of lube oil/hr, that equates to 0.025 gal/hr, which produces 23.62 lbs CO2 per gallon (EIA), equating to 0.6 lbs CO2/hr, resulting in a volume of 3.35 ft3 per hour (CO2 density at 30 PSI and 100 °C is 2.87 kg/m3. The CO2 argon mixture is allowed to build up to 15% CO2 concenteation, then vented into the atmosphere and switched to a secondary tank of pure argon, once the secondary tank again reaches 15% CO2, the tank is switched back, and this cycle is repeated every 30 minutes, maintaining a 15% CO2 concentration maximum, with an average of much less. The argon tanks are sized to provide this ratio maximum, and switched every 30 minutes. Argon consumption per hour is 1.6 kg, at a cost of $0.11/kg for liquid argon (Metals and Minerals Trading Corporation of India, 2016) resulting in $0.18/hr cost). The argon density at 30 psi and 100 °C is 2.57 kg/m3.
Thus claims that the closed-cycle “cannot work” due to CO2 buildup from lube-oil consumption are incorrect. P.C.T De Boer, the first person to experiment with a closed-cycle hydrogen engine, used the same solution for dealing with CO2 buildup. A small 1 cubic foot (28 liter) tank at 35 mpa, will store enough argon for 8 hours of driving. Thus argon consumption, or CO2 build-up, is a non-issue.
Closed cycle working principle
Closed-cycle engine illustration
Drawing to illustrate the small oxygen tank requirement
Dual liquid hydrogen dewars providing 650 miles (1050 miles) of range ©Ivo Jardim
Mechanical-Hydraulic actuated 200 Bar hydrogen injector
Direct hydrogen injection
Liquid hydrogen dewar