11 Nov 2024

The need for clean, renewable energy sources requires exploring carbon-neutral fuels and their combustion behaviors. This is typically done using single-cylinder (SC) engines. The advantages of this process are to make quick hardware changes such as replacing the head or piston, or to change the fuel composition, which provide fuel cost savings compared to a multi-cylinder engine.

Combustion control strategies, various air-fuel ratios, and the impact on emissions are studied using the simulation platform, GT-SUITE, and its real-time engine plant model solution, GT-POWER-xRT.  

turbocharger interaction

Control strategies applied to the SC cannot be applied one-to-one for the MC engine

Based on this research, multi-cylinder (MC) engines are designed, simulated and manufactured. The main problems that can result from this methodology are the differences in behavior between a single-and multi-cylinder engine due to the cylinder-to-cylinder and turbocharger interaction.

These interactions are not represented in the SC engine. Therefore, control strategies applied to the SC cannot be applied one-to-one for the MC engine.

MC simulation results

To mitigate this problem, engine simulations of a MC engine using combustion data from a SC engine are carried out to test and develop control strategies in the time before the multi-cylinder engine is built and available on a test bench. The other drawback is the time between taking SC measurements and applying it to the MC model, which can be weeks or even months. 

It is not uncommon to find issues or at least determine some data are questionable after analyzing MC simulation results. If possible, SC measurements are taken again, or the project is continued based on assumptions that might or might not be good.

Using Simulation to Model Varied Engine Configurations  

A solution to problems is running the MC model in parallel and in real time when measuring SC data

A solution to both problems is running the MC model in parallel and in real time when measuring SC data. The real SC engine provides the combustion data, which then can be applied to all cylinders in the MC model.

The differences in gas exchange for each cylinder, such as varying trapped gas and residual fractions, are captured, and the same is true for the interaction with the turbo.

Fuel composition and air-fuel ratio

The MC provides engine speed, crank-angle resolved intake/exhaust pressures, and average intake temperature. The SC needs to be equipped with fast-acting valves (e.g., 10 kHz) on the intake and exhaust side to impose the conditions that come from the MC. 

Similarly, changes in fuel composition and air-fuel ratio (AFR) can easily be studied, and control strategies for the MC can be developed. 

Why are Carbon Neutral Fuels Different? 

Combining measurements and simulation for combustion/control developments is especially interesting for hydrogen/natural gas or methane blends. Combustion characteristics like the laminar flame speed strongly depend on the actual concentration, especially for hydrogen.

Hydrogen’s ability to burn at very lean conditions, combined with the fact that nitrogen oxide (NOx) formation reaches its peak at relative air-fuel ratios (‘lambda’) of ~2.0, make it useful to run at quite lean conditions.

charging system requirements

The charging system must be able to deliver high boost pressure levels with low exhaust energy

Current trends in engine development are finding that operation at lambda 3 is not uncommon and some research indicates that this could even go higher. This requires a different approach determining the charging system requirements compared to conventional fuels like gasoline, diesel, or natural gas.

The charging system must be able to deliver high boost pressure levels with low exhaust energy due to low combustion temperatures caused by excess air. 

Power generation applications

Therefore, optimized turbos, electric turbo (eTurbos) and/or electric compressors (eCompressors) are considered, especially for on-highway applications.   

For power generation applications, the time-to-torque is essential. Coupling SC and MC enables control strategy development accounting for transient effects like turbo lag or fueling for non-direct injection (DI) applications. 

HiL Systems to Run the Real-Time Model 

Depending on the test cell infrastructure, the MC model can be executed on the test bench machine

Depending on the test cell infrastructure, the MC model can be executed on the test bench machine. There is no need for a HiL system.

The MC model can be linked directly to ETAS INCA, Vector CANape or any system simulation tool that supports FMUs, like Synopsis Silver. If combustion data are not already available from the SC test cell software, three pressure analyses (TPA) in GT-SUITE can be integrated into the process. 

MC model cylinders

A TPA model typically consists of a single cylinder representing the test cell hardware. Dynamic intake, exhaust, and cylinder pressures are used as model inputs.

For this application, intake and exhaust pressures plus intake temperatures are extracted from the MC model. The output of the TPA model is a burn rate that describes how fuel and air burns. This combustion profile can be directly imposed in the MC model cylinders. 

Combustion and Controls Simulation Capabilities 

If they are interested in applying this technique to the development process or have questions on the process, please contact them for specific comments or questions. Learn more about GT-SUITE and the propulsion systems applications.