HEDGEing our bets on the Internal Combustion Engine
Previously HYE discussed that batteries and electrification offer attractively low levels of CO2 if the emissions boundary is ONLY drawn around the vehicle. One realistic barrier to wider adoption of battery vehicles is the high cost to design and produce a battery vehicle, which is then passed on to the consumer as a higher price. To illustrate just how expensive a battery electric vehicle (BEV) currently is, consider the following example.
The Chevrolet Bolt (Figure 1) is the most efficiently priced battery electric vehicle at $61 per kWh (1). It is also advertised as the first affordable EV with greater than 200-mile range. A quick experiment on Chevy’s website suggests that after tax, title and license you can purchase one for around $40,000, yielding payments (60 months, 3.11% APR) of around $800 a month once insurance is factored in. Many sources (2,3,4,5) suggest that a responsible level of financing for a vehicle (including interest and insurance) is around 10-15% of gross income. So, to afford an $800 a month car payment, you need to earn between $64,000 and $96,000, annually. For reference, the median household income in the U.S., as reported in September 2017, was $59,039 (6). Note that this assumes the BEV would be the sole household vehicle. By the same method, a Tesla Model 3 would require an income between $72,000 and $108,000 and for a top-spec Tesla S P100D, that target income is around $300,000, between the 97th and 98th percentile of household income in the U.S. HYE founder Kelly Senecal went on a fact-finding mission to find some high-efficiency internal combustion engine (ICE) only vehicles. Unsurprisingly, the target salary range needed to afford even the most expensive vehicle in that mission (Toyota Camry) is under the U.S. median income at $37,000-$56,000.
To make a long story short, ICE powered vehicles will be required for a long time to come as a stand-alone powertrain or with hybridization. Improving their efficiency is paramount to realizing real global greenhouse gas (GHG) reduction in the future. However, before we look forward at today’s efforts to make engines more efficient, let’s look back.
The late 19th and early 20th centuries were pioneering times for researching the internal combustion engine. I occasionally look back at seminal publications from around this time and am constantly in awe of what they achieved, as well as being envious that they were starting out on a 100+-year journey. One such paper published in 1906 mentions an indicated thermal efficiency of 35% (7). For perspective, I imagine all production gasoline engines today have a ‘peak’ indicated efficiency greater than 35%, however not many mass-volume production engines could claim 45%, if any. So, that doesn’t sound like significant progress considering the 110+ years which have passed since the 1906 paper, does it? Before being too disappointed we need to put that number into context. We should consider that over the same time, engines have become more powerful, more refined, and have higher efficiency over larger areas of engine operation. To look at the barriers to higher peak efficiency we need to go back to basics.
The textbook definition for Otto-cycle thermal efficiency is a simple one. Increasing expansion ratio and ratio of specific heats wins the day (Figure 2). The subtleties of realizing an actual benefit in the test cell is the real challenge for engineers today. Finding a gain somewhere in the system is usually at the expense of a loss somewhere else, negating the benefit. Other times, an unacceptable problem is encountered, such as a noise, vibration and harshness (NVH) problem.
To make efficiency strides, new and novel approaches are required. A group of engineers and technicians from Southwest Research Institute (SwRI) in San Antonio, Texas are performing detailed studies into system gains and losses to identify potential strategies for thermal efficiency improvements. The consortium program is known as HEDGE (High-Efficiency Dilute Gasoline Engine). HEDGE’s origins can be traced back to 2003, where the program focus was on the application of cooled exhaust gas recirculation (EGR) to large displacement gasoline engines. The motivation was to offer a competitive technology to heavy-duty diesel engines while using a low-cost, three-way catalyst, after-treatment solution.
Today, HEDGE is funded by 26 automotive industry stakeholders with a common goal, to enable high-efficiency combustion systems for future powertrain portfolios. The 26 invested companies are comprised of OEMs as well as Tier 1 and Tier 2 suppliers for the automotive sector, who bring a combined research budget of around $2.5 million per year.
HEDGE has a target of achieving 45% brake thermal efficiency. In addition, it must also achieve 75 kW/L power density under stoichiometric operation which can enable SULEV 20 as well as Euro 7 and RDE compliance. These are lofty challenges which demand new ways of approaching the problem that engineers have been working on for over 150 years.
Aggressive targets require aggressive research. So, what exactly is HEDGE working on? HEDGE is currently focusing on five major tasks in the program.
The sound of an engine knocking can be polarizing. On the one hand, it is a clear sign that the engine has unfortunately reached a limit in terms of efficiency. On the other hand, it is a clear sign that the engine has achieved the limit in terms of efficiency for a certain set of constraints! A future article will go into further details about this phenomenon but for now, let’s just consider it a bad thing. While HEDGE will try to reduce the occurrence of knock, it is certainly not the first program to investigate engine knock.
In 1876, Sir Dugald Clerk built a 3.7 L single cylinder engine which exhibited a high frequency pressure oscillation accompanied by a distinctive sound (Figure 4). Unperturbed, in 1885 he commissioned a larger, 23 L displacement engine. The audible knock from the larger engine could be heard around 700 feet away. Interestingly, they determined that injection of water would mitigate the knock significantly. Fast forward 140 years and water injection is being researched with renewed interest (8).
While it is perhaps unrealistic to target the complete abatement of knock in HEDGE, making any improvement yields higher efficiency and is therefore a key area of study. Along with traditional knock mitigation strategies, e.g. water injection, EGR, reformate-rich EGR and EIVC/LIVC etc., HEDGE is looking into the fundamental effects of coatings on knock and efficiency. Coatings can broadly be split into two categories, thermal barrier coatings (TBC) and thermal swing coatings (TSC). Currently, coatings are being screened using a 3-D CFD, 1-D CHT coupled simulation. Promising coatings are then applied to engine components using facilities at SwRI before testing on a single-cylinder engine test platform (Figure 5).
Microwave Enhanced Combustion
Putting things into a microwave and seeing what happens is something to discourage in children. For combustion engineers, it can lead to new areas of interesting research. To that end, HEDGE is researching the effects of coupling MW energy to flames. By coupling MW energy to a flame-front, radicals are produced which enhance the flame. In the images below (Figure 6), the development of a tealight candle flame when coupled to a MW flame can be seen. HEDGE has patented a type of antenna for use in light-duty gasoline engines.
Along with fuel type, the ignition event separates the diesel engine from the gasoline engine. Tens of thousands of volts ionize gas to several thousand degrees Celsius. This process sets off an exothermic chain reaction releasing the chemical energy held within the fuel. For many people it is taken for granted; even the most sophisticated combustion models typically just assume combustion has started at some prescribed point in time. With an increase in cylinder pressures and dilute mixtures, the ignition process has never been more important to understand (and model). HEDGE is developing a 3-D ignition model which models the entire secondary circuit of an ignition system and accounts for variability in turbulence and flow motion. Results are validated against optical measurements taken in a bespoke designed optical rig where high-speed images (up to 1,000,000 fps) have been obtained. A multi-cylinder engine with optical access is also being used to correlate an ignition event to the following combustion process.
Gas Separation Membrane
It is common knowledge that air, consumed by internal combustion engines, consists of approximately 78% Nitrogen 21% Oxygen and 1% Argon (traces of many other elements also exist). This species mix yields a ratio of specific heats of 1.4 at room temperature. The ratio of specific heats determines how much work can be extracted from the gas during expansion as well as the in-cylinder temperature during compression. Hence, a higher gamma will yield a higher efficiency (Figure 2). That’s all very interesting but we can’t really do much about the composition of air… or can we? HEDGE is looking at just that by studying combustion perfomance with varying gas composition as well as finding ways to practically change the composition on the engine.
SwRI is funding a study to look at a passive, ‘zero’ pressure loss membrane (US Patent 8,454,732 B2) which can selectively remove only CO2 from the exhaust stream. The potential benfits of this technology are as of yet unknown, but the possibilities are endless and part of continuing research.
A new task to HEDGE is the ‘Systems Evaluation’ task. While a high efficiency internal combustion engine is the goal of HEDGE, the true value to the industry is lower GHG emissions. Therefore, the new task will look at synergies between different technologies, how well they fit with different combustion modes and, most importantly, what benefit they give over homologation drive-cycles. Taking this a step further, vehicle modeling will try to find complementary technologies for different levels of hybridization for different vehicle sizes. A new VW-Audi EA888 Gen 3 b-cycle engine will be used to compare different technologies and generate data for vehicle simulation. The engine is currently being installed in San Antonio and as it hasn’t yet turned a crank in anger, it seems like a perfect time to give it a hug!
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 Clerk,D. ”On the Specific Heat of Heat Flow from, and other Phenomena of the Working Fluid in the Cylinder of the Internal Combustion Engine”, Proceedings of the Royal Society, vol 77, pg 500, Received February 24, Read March 15, 1906.
 Clerk,D.,Cylinder actions in gas and gasoline engines, SAE 210043, 1921.