A Highly Efficient, Compact and Affordable Engine

After months of analyzing test data, improving simulation code to better reflect what’s happening in the real engine and some design improvements, The ultimate performance of the Clarke-Brayton engine keeps getting clearer – and for the first time we can confirm that it can do all of this while meeting stringent NOx emissions standards using conventional aftertreatment systems.

A recent design improvement from the mind of Chief Scientist and inventor John Clarke improves power density even further:

On the left is a 359 horsepower Clarke-Brayton V6 compared to a 325 horsepower Cummins 6.7L on the right

On the left is a 359 horsepower Clarke-Brayton V6 compared to a 325 horsepower Cummins 6.7L on the right

Above is a comparison of a Clarke-Brayton Engine in a V6 configuration on the left putting out 359 horsepower compared to a 325 horsepower Cummins 6.7L I6 – both engines are compared at the same mean piston speed. In addition to the clear advantage we have in size and weight, the reduced amount of material will also reduce cost.  Further, the Clarke-Brayton Engine is naturally aspirated so the expense of turbos and aftercoolers are eliminated and only 1/3 the number of fuel injectors are required leading to considerable cost benefits.

A naturally aspirated engine has dramatically improved transient response, meaning when you depress the accelerator, the engine responds with more power and speed immediately, eliminating the so-called “turbo lag” suffered by virtually all conventional diesels.

The engine also shows a remarkably flat torque curve and extremely efficient operation at all conditions, as shown in the indicated thermal efficiency map below.

Indicated Thermal Efficiency Map of the Clarke-Brayton Engine

Indicated Thermal Efficiency Map of the Clarke-Brayton Engine

Peak indicated thermal efficiency is 59% (for you engine nerds out there, this includes gas exchange/pumping losses). More impressive is that efficiency remains well above 53% even in low-load, low speed conditions where vehicles spend most of their time operating.

Our friction model predicts peak brake thermal efficiency at near 55%, compared to 42% for today’s best automotive diesels.

This engine promises to have great benefit to a number of applications including heavy-duty and medium-duty trucking, automotive, marine and power generation. It’s ability to use natural gas as a compression-ignition fuel – the subject of a future post – further increases its attractiveness in a number of segments.  We’ve been talking to a number of interested potential strategic partners and are excited about finding the perfect relationship to help propel this technology to market.

Motiv Releases Design of MkII Clarke-Brayton Engine

I have been woefully absent from these pages the last several months since we started the design effort on the new engine. I am excited to finally be able to share what the team has been working on so dilligently.  The MkII Clarke-Brayton Engine is the next step in dramatically reducing fuel consumption in trucks, automobiles and generators without increasing costs.  It is also the next step in developing a highly efficient compression-ignition 100% natural gas engine that can meet the up-coming greenhouse gas emissions regulations. This is a boxer configuration split-cycle engine implementing what we have come to call the Clarke-Brayton cycle.  The thermodynamics of this engine are virtually identical to our previous “CCI” design but are implemented in a much more conventional way.  Everything that we published in our SAE paper at the 2013 World Congress holds true for this engine, but many of the difficulties related to the old engine are resolved.

MkII Clarke-Brayton Engine

MkII Clarke-Brayton Engine

Section of the MkII engine

Section of the MkII engine

As with the previous design, in the MkII air moves sequentially through three cylinders, starting at the mid-sized cylinders at the top of the section image. This architecture allows us to achieve a 56:1 compression ratio leading to a 30MPa peak pressure.  It has far less surface area for heat loss than a comparable conventional diesel due to the very small bore of the combustion chamber. The small combustion piston area leads to lower forces on the crank than a conventional engine would have if it were able to reach similar pressures, reducing rod bearing friction compared to conventional architectures. A lack of net forces on the main bearings due to the opposing forces of the piston pairs reduces main bearing friction compared to conventional engines. It expands exhaust gasses all the way to ambient pressure before the exhaust stroke.  Gas transfer from one cylinder to the next is begun at equal pressures on either side of the valve, which keeps velocities low, minimizing pumping losses and eliminates blow-down. The power is produced in almost a 50-50 split between the combustion (central) and exhaust (largest) pistons.  There is a power stroke every revolution. All valves are actuated by overhead cams. Piston ring sealing is completely conventional, eliminating the dynamic effects of the old design and greatly reducing the reciprocating mass.

The major components of the MkII Clarke-Brayton Engine have already been released to the foundry for casting and everything else should be released for fabrication within a couple of weeks.  We will test this summer at a globally renowned engine development lab and I hope to have results to share shortly after that.

A team of just three people designed the MkII from a back-of-the-napkin idea to a fully developed test engine in 7 months.  Azra Horowitz and John Clarke have both put in herculean efforts to get this done in time despite a couple unexpected thorny technical challenges along the way.  I could not be more proud to be working with them.

Volvo’s new E-DRIVE Diesel with i-ART fuel injection

This article caught my attention from a tweet by SAE on the debut of Volvo’s new diesel engine. The main advancement described in the article is a more accurate closed-loop control system for fuel injection that includes a pressure sensor in each injector rather than just a single high-pressure rail sensor.

Volvo iARTThis quote from the article confuses me as to how big a breakthrough this actually is:

Fuel consumption of the new engines is reduced by 10-30% compared to units of similar output but of larger capacity, Crabb claimed. Denso engineers have stated that i-ART can improve fuel efficiency by 2%, compared with open-loop systems.

So is this a truly revolutionary improvement or another small incremental step? It is unclear from this quote, but my guess is that under certain conditions, they are improving efficiency by 2% – sometimes. So if you start with 40 mpg, this system could improve it to 40.8 mpg under certain conditions.  Of course this is only a guess.

Something else that I found very interesting is that they are using ball bearings for the cam shafts.  This can clearly have benefits on reducing friction, but ball bearings have always been avoided on engines due to a lack of load capacity and longevity.  I wonder how they are solving this problem!  Knowing Volvo, I am sure that these bearings have been tested six ways to Sunday.  Interestingly enough, in our first CCI prototype we used ball bearings as main bearings because we were not overly concerned with longevity in that lab-only prototype.

Is a Tesla more efficient than a conventional car?

Tesla motors has been a shining beacon for electric vehicles, rising to prominence whilst most of their brethren buckle under financial, technical and market problems.  Tesla has been able to produce an extremely high quality product as evidenced by its receipt of the highest possible grade by Consumer Reports.  The savvy and patient management by Elon Musk has allowed the company to make it through many inevitable challenges without running out of cash to make the big commercial push.  Most people assume that not only is the Tesla a sexy high performance sports sedan, but also that it is extremely efficient and great for the environment because it is electric.  But is it?

Is a Tesla more efficient than a gasoline car?

Is a Tesla more efficient than a gasoline car?

How does one measure efficiency?  Simply put, how much of the energy available in the fuel is turned in to kinetic energy to move the car?  In a conventional car, this is how much of the chemical energy contained in gasoline is turned into rotational energy by the internal combustion engine.  In modern cars, this turns out to be about 30%.  But this is the best efficiency point of the engine.  A more realistic point to choose is 25%. To figure it out for the Tesla (or any grid-charged electric vehicle) is a little more complicated.  First you must find out the efficiency of the power plant generating the electricity, whether it is coal, natural gas, nuclear, solar or wind.  From 2000 to 2009 the overall efficiency of US power generation increased from 35.5% to 40% thanks in large part to a 50% increase in the use of natural gas at the expense of coal, and to a lesser extent, the increase in renewable sources of energy which still account for only a minor fraction of the energy mix.  Next, there are losses involved in the transmission of the electricity from the plant to the charging station for the electric car.  The DOE estimates these losses to be 9.5%.  Next, there is the charging and discharging cycle for the batteries, estimated by the DOE at 85% and 95% respectively.

The results:

Efficiency of the Tesla: 29.2%

Efficiency of a gasoline car: 25%

And for self promotional purposes, the efficiency of a vehicle powered by one of our CCI engines: 52% (or with waste heat recovery, significantly higher still)

What does this mean?  Well, the cost of fuel to the end user of a Tesla is a lot less than that of a gasoline car, but that is at a huge up-front expense that you are unlikely to recover over the life of the vehicle.  Is there a marked difference to the environment?  That takes another analysis regarding the emissions of different kinds of power plants vs. gasoline engines (and then merits looking at natural gas powered vehicles!).  But the takeaway should be that while the Tesla Model S is a great car, if your motivation for purchasing it is to save the planet, you’ve got to think a little deeper before writing the check.

Note* this article was modified from it’s original by using an efficiency less than the best point for a gasoline engine as a more realistic scenario.  Thanks to SuperDuper from MotorTrend forum.

Liquified Natural Gas (LNG) Trucks – Benefits, Challenges and Solutions with the CCI Engine

We are all familiar many reasons for transitioning from gasoline and diesel in the transportation sector to alternative power sources – the environment, national security, and economy to name three.  Greenhouse gas emissions controls are expected to go into effect in the near future in the US and there are very few options conventional engine manufacturers have for lowering GHG emissions other than making more efficient engines – an effort which is becoming more and more difficult.  Natural gas has lower GHG emissions than diesel or gasoline at the same efficiencies so it can be a solution to meeting these upcoming federal regulations. The relatively recent unlocking of massive amounts of natural gas reserves gives us a compelling candidate fuel.  Adding to the advantages above, the massive domestic supplies of this fuel mean it is much cheaper, giving it potential to reduce US_Natural_Gas_Productionshipping costs and therefore reducing costs of all sorts of goods people rely on.  But it is not without challenges.  Compressed natural gas (CNG) is limited in range because you cannot store a lot of fuel on board a vehicle.  LNG allows for greater range, but it must be kept in cryogenic tanks.  If the fuel is not used quickly and warms up too much, some must be vented to the atmosphere which is bad for the pocket book as well as the environment.  This characteristic makes it a difficult choice for regular passenger vehicles which may sit in a garage for several days without being used, but it is an excellent choice for trucking when one can be certain that the vehicle will be on the road every day and is responsible for 15% of our use of oil and 75% of the oil we import.

There is already some serious effort going in to increasing the size of the national LNG fleet.  Cummins Westport is offering an ever-wider selection of high quality engines.  With the support of natural gas super-promoter Boone Pickens, Clean Energy Fuels is building out a network of LNG fueling stations – a key prerequisite to making more LNG trucking a reality.  The low cost of the fuel makes it very attractive as a method of lowering shipping costs, but it comes at the price of higher up-front costs.  The engines and fuel tanks are expensive, causing trucks to cost approximately $100,000 more than diesel trucks.  This is a significant hurdle.  The New York Times recently published an interesting article on this subject.

As I outlined in an earlier post, the CCI engine has an inherent advantage to the natural gas engines available today. Prices may have moved a bit since that post, but the cost advantages remain very compelling.  To recap, a conventional spark-ignited LNG engine has a brake thermal efficiency of around 27% whereas we expect that we will be able to build a CCI engine burning LNG with a brake thermal efficiency in excess of 50%. Therefore operating a diesel truck costs $0.18 per horsepower hour, a spark-ignited LNG engine $0.14 and a CCI engine $0.07.  In addition, an analysis on full-production costs of the CCI engine performed by Caterpillar Inc. when the technology was still under their umbrella put it in line roughly with diesel engines, lowering the up-front price required for a fleet to purchase a natural gas truck.

The CCI engine used in a LNG truck can make a huge impact on the costs of operating a truck fleet and therefore the costs of almost all goods transported in the US.  It can significantly reduce the risk we face with our reliance on foreign sources of energy and it can provide a method of reducing greenhouse gas emissions on an economically beneficial basis.

Engine Efficiency and Thermodynamic Cycles

A comment on my post announcing the first firing of the CCI has inspired me to write a bit about what kind of inherent limitations different engines have and how the CCI’s theoretical limitations are higher.  I will spare you the Pressure-Volume diagrams we love to look at here at Motiv.  Instead, we’ll go straight to the numbers computed from those diagrams.

The engine Americans are most familiar with is the gasoline automobile engine.  This engine uses what is called the Otto Cycle.  Then there are diesel engines which predictably use the Diesel Cycle.  Additional cycles are Atkinson, which is similar to Otto but expanding exhaust all the way to atmospheric pressure before sending it out the pipe, and Brayton, the cycle used by jet engines and our CCI, which has the expansion of the Atkinson Cycle, but the constant pressure combustion of the Diesel.  Finally, there is the Carnot Cycle, the cycle that theoretically is the most efficient but engineers have struggled to implement this cycle in a real engine that is affordable, practical to use and is able to maintain the high potential efficiency.  If we try to compare these cycles against each other, it is useful to start out doing this at the same peak pressure ratio so we can see the effect on efficiency of cycle only.  In the real world, the differences will be even more dramatic because the Otto and Diesel cycles generally must use much lower pressure ratios for practical reasons.  In this chart, we compare all cycles at the high pressure ratio used by the CCI Engine.

The efficiencies in this table are theoretical maximums, assuming the engines are all frictionless, have no heat loss, 100% reversible combustion and do not have any accessories like oil pumps, alternators, etc.  All of these things reduce efficiency.  You can see that an engine using the Brayton Cycle has an inherent advantage over gasoline and diesel engines that have a much lower theoretical limit.  Consider, however, that there are practical limits to the compression ratios that a diesel can use which would further limit it to around 56%.  The Otto Cycle has even greater constraints on compression ratio that limit it’s theoretical efficiency even more.  A note about jet engines using the Brayton Cycle: their turbines are not cabable of generating the same kind of pressure ratios that the pistons of the CCI can, which limits their efficiency compared to the CCI.

There are a lot of new engines out there such as Achates Power, EcoMotors and Pinnacle Engines that are working on alternative architecture engines claiming higher efficiency, but they are all working off the same old LIMITED cycles.  This makes their claims on significantly higher efficiency hard to believe because thermodynamically these engines are not different from ones that have been built millions of times before.  Some of them base claims of higher “efficiency” in a car by shutting down some cylinders during highway use or combining them in hybrid systems.  I find these claims to be very misleading because it has nothing to do with the efficiency of the engine and is a strategy that can be used by ANY engine to reduce the fuel consumption of a vehicle.  Then there is the Michigan State University Wave Disk Engine that has claimed a 400% to 500% improvement in efficiency.  A professor of mechanical engineering at a prestigious university using such absurd and obviously false hyperbole to obtain an ARPA-E grant makes me cringe.

I try to always be accurate and objective when talking about the CCI engine and perhaps that gets lost in the weeds of the stretched claims by many of my competitors.  Thankfully, I think even against many of the unrealistic stats they publish, our realistic ones still look a lot more attractive!

The CCI Engine and a Gas Self-Sufficient US

On November 13th, the EIA forecast that the US would become a net exporter of natural gas by 2022 as reported by Reuters.  The US is using more natural gas for power production, and even transportation fuel, as the recently reported T. Boone Pickens Clean Energy Fuels Corp. deal with GE highlights.  All of this is happening even though there are a number of drawbacks for natural gas engine fuel storage systems because the economic, geopolitical and environmental benefits to doing it are so strong.  This gives some certainty that natural gas will play a major roll in the portfolio of future North American energy resources.  I find this all to be very positive not only for our country, but for the CCI Engine.

I have written before about the benefits of the CCI running on natural gas.  Its ability to use high compression ratios and direct injection without requiring a pilot injection of diesel or some other ignition aid makes the efficiency and cost of the CCI leaps and bounds ahead of other solutions such as the Cummins Westport engines.  There are even possibilities that the CCI could use natural gas as a Homogeneous Charge Compression Ignition (HCCI) fuel for a really low-cost low-emissions natural gas engine.

I think the move to natural gas makes the already attractive CCI engine even more attractive.  It’s benefits in efficiency and power density hold just as true for natural gas as they do for diesel or biofuels.  With natural gas we have an added benefit over current technology in that just getting close to diesel-like efficiency with NG today requires very expensive engines and dual fuel.  The CCI can exceed that efficiency level with a reasonably priced engine and a single-fuel system.