Learning that a team of engineers decided to build a new ICE, during a period that the norm leans towards electric engines will probably raise quite a few eyebrows. We decided to create this FAQ page in order to offer clarifications to the most frequently asked questions; we will update this page as soon as we have a new question popping up often enough.
Frequently asked questions for the SARM engine
Why another internal combustion engine?
Internal combustion engines are significantly more effective, compared to electric engines, in applications that require (a) extended time of operation (b) heavy duty tasks (c) diverse conditions of operation. The electric engines have a great drawback: batteries. They cannot support vehicles or machinery that will operate for long periods of time, in diverse conditions or performing heavy duty tasks.
What types of fuel does the SARM engine use?
The SARM engine can operate using the following types of fuel:
Will the engine look like the image you use in your website?
The core engine yes. The entire set-up will of course be different - more bulky - as it will bear additional equipment necessary to its operation. In time, as the engine will be optimized, the overall design and external look of the engine will alter as well, becoming less bulky and more disk-like.
Can you give us some more details about how this engine works?
A detailed description of the Operational Principle and the benefits of the SARM engine can be found in the SAE paper “Description of a Novel Concentric Rotary Engine”, S.Savvakis, V. Gkoutzamanis, Z.Samaras, doi.org/10.4271/2018-01-0365
What is the thermal efficiency and how did you calculate it?
The thermal efficiency depends on the engine’s speed. tby using detailed 3D simulations using ANSYS Fluent (during Savvas PhD). For our Attkinson based operating cycle there is no limit for the compression ratio (CR) value. It can be as high as possible, which means also a higher efficiency. For a CR of 13, the SARM engine is 19% more efficient than the SI engines (paper “Numerical simulation of a novel rotary engine compared to conventional reciprocating engine cycle”, V. Gkoutzamanis, S.Savvakis, A. Kalfas, Proceedings of Shanghai 2017Global Power and Propulsion Forum, 2017, GPPS-2017-147). However, this can be even higher, because Mazda Skyactiv engines have reached an CR of 14 and they claim to go higher than 16.
SARM thermal efficiency: 36-42% (100rpm - 5,000rpm): based on detailed 3D simulations created in ANSYS Fluent. These simulations include all processes of one operational principle: induction, compression, injection, vaporisation, ignition, combustion, expansion and exhaust gas remove process. However, we run only 2 cycles in a row and with random design parameters. It was very difficult and time-demanding to operate the whole cycle more than two cycles (dynamic mesh). On the other hand, as expected, we saw that the second cycle showed a higher efficiency than the first one. This is going to happen until the phenomena will stabilise. Our experience with our compressor showed that the first can be up to 15% higher than the 5th or 6th cycle. Therefore, we are going to simulate the operating cycle also with CovnergeCFD where the model setup is much faster because we don’t need to create a mesh. Our goal with ConvergeCFD is to run in a row 5-30 operating cycles (until the thermodynamic conditions do not change between two cycles) and then optimise the design parameters to find out the optimised theoretical thermal efficiency of the engine for a speed range from 1000rpm to 15,000rpm.
The same CFD simulations concluded that the combustion process is much faster because the turbulence is much higher than in conventional engines. The higher the turbulence, the faster the completion of the combustion process. This is also confirmed by the Toyota's Dynamic Force Engine where they claim that this engine creates a strong tumble flow that gives a 30% faster combustion process and a lower energy loss. This also answers the question of some engineers who say that if we have higher turbulence, than we have to expect higher cooling losses. Toyota’s engine has a higher turbulence but the efficiency is higher because the combustion process is much faster than the cooling process.
What is the pressure loss caused by the sealing of the engine? Wankel engine has a very bad reputation for its seals.
We don’t use apex and side seals like Wankel. use labyrinth sealing and the efficiency of this sealing depends on the engine’s speed & the size of the engine. The higher the engine speed, the lower the working medium that can escape through the labyrinth. The bigger the engine’s size, the longer the leakage path & smaller the gaps compared to the engine’s size. Simulations showed that we can have up to 93.5% compression isentropic efficiency. That means we can achieve a pressure at the end of the compression phase 93.5% of the one we would achieve if the compression process was isentropic. (Master Thesis D. Vogiatzis, Z. Samaras)
As far as the sealing between the piston and compression chamber, Master Thesis of K. Zoubourlos concluded the following diagram, in this 3D CFD study:
Why do you believe that SARM can be up to 4 times smaller and lighter?
Its power density can be up to 4 times higher compared to the 4-stroke engine, because SARM is able to give one power stroke every 180 degrees.
What are the cooling losses? Your Surface to Volume ratio is very high, compared to the reciprocating engines, even greater than Wankel’s ratio
Our high Surface to Volume ratio (S/V) makes many people believe that we are going to have much higher cooling losses than the conventional engines. Their assumption is logical and expected because the Wankel engine has a high S/V ratio and this causes its “cold flame” during the combustion process. The main reason of the low thermal efficiency of the Wankel type engines. However, our combustion process completes in less than 3 degrees (compared to 15 to 35 in the case of other SI engines) and it takes place when the S/V ratio of the combustion chamber has the same value with the S/V of a sphere. Therefore, the pressure loss are very limited, more limited compared to the reciprocating engines as well. During the expansion process, after reaching the peak pressure of the combustion process, the pressure and temperature are going lower and the S/V is going higher. You can read the afore-mentioned SAE paper for more details on that.
On the other hand, during the expansion process, the exposed surface of the working medium to the cooling system is significant, but the ΔP & ΔT between the working medium and the environment is smaller. The fast expansion of the initially burned volume generates a lower peak-pressure & -temperature (50bar & 1150K) compared to the peak-pressure & temperature of other engines (70-100bar & >2000K). Therefore, during the expansion process we have also a smaller ΔP & ΔT compared to the other engines’ types.
What about the NOx emissions? Why do you talk about 80% less NOx?
The fast expansion develops lower temperatures (peak temperature 1150K). Therefore, we avoid the generation of NOx.
Are there any assumption regarding your benefits that are not yet tested or calculated?
For the noise is responsible the combustion process, the induction process and the remove of the exhaust gases through the exhaust port. The higher the engine’s speed, the higher the noise, but in any case a totally different and smoother noise than the reciprocating engines. Wankel type engines are also more silent than reciprocating engines.
What types of fuels does SARM use?
SARM can use all kind of fuels apart from Diesel. At the moment, we didn't test SARM as a Compression Ignition engine (Diesel type engine). Moreover, by using hydrogen, syn-fuels, e-fuels, bio-fuels, SARM can be characterised as a zero emission engine as well.
The velocities developed during the transfer of the compressed working medium from the compression chamber to the expansion chamber can be limited by the Mach number. Is this true?
Even though the velocities that can be developed inside the pressure chamber can be more than 340m/s, the fluid is not throttled during its transfer due to the Mach number because the Mach number depends on the temperature and pressure and it becomes higher when these two thermodynamic conditions increase. Therefore, the Mach number does not exceed the value of 1 during the transfer of the working medium.
Why use variable CR?
High-powered multi-cylinder internal combustion engines may be necessary to satisfy driver demands for quick acceleration and/or heavy towing capacity. However, during daily use, they generally operate at power settings of less than 25% (e.g. at freeway speeds, less than 40 hp (30 kW) are required to overcome aerodynamic drag, rolling friction, and to operate accessories such as air conditioning).
However, when a gasoline internal combustion engine is operating under less than full load, the effective compression ratio is much lower than the measured (geometry based) compression ratio because the throttle is not fully open, and therefore the cylinders receive less than a full charge of air at each intake stroke
Why do you use steel and not aluminum in your structures?
Our competitive advantage in weight allows us to use steel instead of aluminum because the total life cycle CO2-foot print of primary aluminium is more than three times higher than steel. One of the reasons why is this happening is that aluminium has a more complex recycling process compared to steel.