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Grail Engine - A Critical Look
A non-marketing driven inspection of the Grail Engine technology and problems likely associated with it

By: Bruce Bahlmann - Contributing Author (your feedback is important to us!)

Created: April 4, 2012

Recently, I read with great interest a story about the promising qualities of the Grail Engine as explained in an online how to article (how does the grail engine work). In fact, I was so taken by some of the concepts of this engine that I went on to read more about it on the grail engine company website. This article represents a collection of my thoughts and reactions as I read about the Grail Engine with a sprinkle of my life long fascination with the combustion engine for good measure - the aim here is to provide a non-marketing focused interpretation of this technology.

"Promising" benefits of the Grail Engine

The Grail Engine leverages a number of innovative technologies (I will discuss later) to achieve sizeable horsepower, torque, fuel economy, and fuel diversity - all within a very compact space that requires little (if any) cooling that normal 4 stroke engines of similar horsepower and torque require. Below are the specifics from various sources on the Internet detailing the current working prototype as well as larger versions of the Grail Engine that are currently being engineered.

Version 500 cc
1,000 cc
(under development)
2.5 Liter
(drawing board)
Horsepower 100 hp 200 hp ?
Torque 65 ft lbs (@1,500 rpm) 180 ft lbs (@? rpm) ? (@? rpm)
Max Torque ? (@? rpm) ? (@? rpm) ? (@? rpm)
Economy 100+MPG 100+MPG ?
Weight 30 lbs ? ?

Of particular interest in the above is the surprising amount of torque at what appears to be a low (1,500) rpm.

Reached for Comment

I did reach out to the company in an attempt to verify the information I found about the various engine versions on the Internet (this was how I completed the chart above), however when the company called me back they simply stated, "We are in phase three of the process of building our proprietary engine and do not divulge such information as torque, etc." In response, I proceeded to tell them "Actually, you have some information on your website and that other sources have quoted you as giving out some of this information in the past. So, why not share it with me?" The company response to this statement was simply, "This is not the case and that we do not give out that information." So, rather than argue with the guy, I just told them "Perhaps you need to review your website because some of this information is definitely in there." It was a brief exchange, but I cannot help but feel disappointed with how this was handled. Who are these guys and why would they get so defensive? Probably a topic for another article or for someone else to further investigate the company - my sense is that something is definitely amiss.


The Grail Engine claims to not rely on a single breakthrough technology rather simply combines the use of several "proven" innovations to achieve these significantly compact performance figures. These technologies include:

Technology: Description: Image:
Direct Injection
Fuel Injection

Direct Injection provides up to 50% fuel savings over traditional carburetor engines.
This technology involves spraying fuel directly into the firing chamber as opposed to using a traditional carburetor which mixes fuel with incoming air. Fuel injection is commonly used in cars today however it is not directly injected into the cylinder.
Forced Induction

Forced Induction provides up to 40% fuel savings over non-supercharged engines.
This technology involves using the engine's exhaust to spin a turbine which in turn spins another turbine to collect and focus large amounts of fresh air to be sent into the engine. Forced Induction is less commonly used (due to its associated costs) but can be found on high performance vehicles (sports cars, luxury sedans, etc.)
Homogeneous Charge Compression Ignition (HCCI)

HCCI provides up to a 15% fuel savings over traditional spark plug ignition while meeting current emission standards.
This technology involves compressing fuel air mixture to the point it combusts. HCCI closely resembles the ignition process that occurs in diesel engines. The Grail Engine uses a variant of this where instead of compressing to the point of ignition, it compresses "nearly" to the point of ignition and then fires three spark plugs to more completely burn the compressed fuel air mixture.

Achilles Heel of the Grail Engine

What I believe is the Achilles Heel of the Grail Engine is its innovative use of a valve atop the piston (this is NOT common among modern day engines) which is uniquely shown via an animation provided by the company. In this animation, the valve atop the piston opens at the start of the upward compression (pre-combustion) stroke to allow fresh air previously compressed (pre-heated) below the cylinder (during the previous power stroke) to enter the combustion chamber to replace the exhaust which is vented out the valve atop of the cylinder head that is opened and closed just prior to the piston valve opening.

The top of the piston is typically a combustion battleground, where unused combustibles (artifacts) wind up and adhere to its surface (see example figures of a traditional piston after extensive use). While the Grail Engine probably does perform quite well when everything is new (most engines do), the Grail Engine would exceedingly become sensitive to the natural build up of combustible artifacts that will not only severely impact its efficiency, but also can lead to malfunction (such as blow-by which is caused when a combustion chamber is improperly sealed by its rings and the resulting combustion is able to forcibly enter the crank case) and or a much shortened life.

Traditional (non-Grail) engines all use pistons, but their valves are placed atop the header (the top of the engine). While nothing prevents a traditional 4 stroke engine from placing an intake valve atop its pistons, the placement of the valve atop the header has advantages - particularly since gravity pulls artifacts from the combustion down onto the piston (thus away from the sensitive valves). As a result, traditional valves would likely require less maintenance than the Grail Engine.

Also, the pistons used in traditional (non-Grail) engines have a fairly thick non-critical surface that combustion can push against, the Grail Engine has the framework of a piston with a valve in the center which is extremely critical to its operation. The ability for this valve atop the piston in the Grail Engine to not only resist combustion artifacts, but also properly seal, and perform flawlessly, creates a crutch on which the whole Grail Engine leans on to properly operate. When (if) this crutch (piston top valve) malfunctions, the whole engine fails.

What I don't like about this aspect of the Grail Engine is the following:

  • The piston top valve represents a moving part which is part of another moving part - odds say this would be the first to fail
  • "When" the piston top valve fails, since it is placed all the way inside the engine, a complete overhaul is in order - very expensive to maintain
  • While bringing fresh air into the crank case would provide unique cooling opportunities, pre-compression, and pre-heating, it also places extreme requirements on the quality of the air entering the engine as any impurities found in the air would find their way into the crankcase and impact the lubrication effectiveness of the oil there - there is a reason why traditional engines keep the crank case separate from the combustion chamber as well as free flowing air. This brings up another point, if the crankcase oil, cylinder walls, and main bearings are exposed to a constant flow of outside air, this probably demands a specialized oil that can withstand constant exposure to air and humidity and or contaminants within the air - humidity being a monumental challenge to solve in my opinion.
  • Finally, utilizing the power stroke to compress and preheat the air below the piston sounds like a good idea, but it also robs some of the energy of the power stroke. Also, with regards to larger versions of the Grail Engine, such compression likely places increased demands on the starter (particularly cold starts).

Inconsistency Found

After all this time spent reading about this Grail Engine, I went back and reread the article that started me down this path (the How To article referred to earlier in this article) which, oddly enough, is linked to from the company website. What is odd about the article, is that there is a error in the way they describe the engine that has be incorrect, but the company must endorse it because they have linked to it. I'll be happy to edit this article and add the name of the person who can identify the incorrect statement - I know there is at least one.

Closing Comments

While the Grail Engine shows some promising engine efficiencies, car makers are well on their way in utilizing improvements on the technologies mentioned within this article to achieve similar if not better and more reliable results. For example Transonic Supercritical (TSCi) Fuel Injection could be used to with very little changes to modern engines to achieve better fuel efficiency than modern hybrids.

I hope all of you who read this article find my critique of this technology useful and interesting. I welcome your thoughts and comments.

The following are other articles I found about the Grail Engine. I did not check those too for accuracy, but have provided them as additional references.

Additional References: Date:  
New York Times 12/17/2009 A Two-Stroke Revival, Without the Blue Haze
Green Car Congress 9/8/2009 Grail Engine Adopts Enerpulse Pulse Plugs for Forced Semi-Homogeneous Charged Compression Ignition in Concept Two Stroke Engine
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