Although the claims uses the terms "combustible gas" and "air" the description uses sometimes the term "gas" when it means "combustible gas". This is confusing as air is a gas as well. For the avoidance of confusion I will consistently use the term "combustible gases" for combustible gases, "air" for air and technically correct "gas" for "air" or "combustible gas".



Claim 1 of Deutsches Reichspatent N° 532



  1. Introducing into an enclosed space combustible, with air mixed gases and air of a different kind prior to their combustion in such manner that a combustion once initiated at one location slows down while continuing from gas particle to gas particle, whereby the combustion products and the kind of air that envelopes them expand by the produced heat and thus release operational power.



Apart from other issues the claim seems to be difficult to assert against a potential infringer. 

 "a combustion once initiated at one location slows down while continuing from gas particle to gas particle" The burden of proof lays with the patent proprietor. So how can the patent proprietor (at that time) show that in an infringer's gas engine the combustion slows down from the beginning and not, as it in reality does, as we know today, moderately accelerates and then slows down. 




Lack of Clarity

Today, very likely such a claim would be rejected by the European Patent Office as it contravenes the requirement of Clarity (Article 84 EPO).


The area defined by the claims must be as precise as the invention allows. As a general rule, claims which attempt to define the invention by a result to be achieved are not allowed, in particular if they only amount to claiming the underlying technical problem. (see T 68/85)

Cited from EPO Guidelines for examination IV 4.10 Result to be achieved


The second part of the fictious communication pursuant to Article 94(3) EPC (examination report) would continue with further deficiencies to be raised:

In relation to the first invention:


Claim 1 defines subject matter vaguely in terms of "an enclosed space". It is clear from the description that the invention relates to a gas engine with a cylinder within gas is expanded. A claim directed to an "enclosed space" therefore is not supported by the description. Consequently, the applicant is requested to amend the claim to be in line with the description of the invention.

The term "air of a different kind" is vague and not supported by the description. The applicant is requested to specify what the term should cover.

The term "It slows down from gas particle to gas particle." is an attempt to define the invention by a result to be achieved. The invention should be defined by technical features that contribute to achieve this effect.

The applicant is invited to file a new independent claim which takes account of the above comments.

At the moment no opinion about the novelty and inventive step of the invention can be established because it is not clear what the technical features are that are claimed.

Independent claims are not in the two-part form in accordance with Rule 43(1) EPC, which in the present case would be appropriate, with those features known in combination from the prior art being placed in the preamble /Rule 43(1)(a) EPC) and with the remaining features being included in the characterising part (Rule 43(1)(b) EPC).

If, however, the applicant is of the opinion that the two-part form would be inappropriate, then reasons therefore should provide in the letter of reply.

Before the Examiner would start to examine if a claim is new and inventive he will require first that the applicant presents a claim that fulfills the clarity requirement. Although we are allowed to reword a claim during examination, there is the limitation of Art. 123(2):

The European patent application or European patent may not be amended in such a way that it contains subject-matter which extends beyond the content of the application as filed.

Therefore already at the stage of filing a patent application it is good practice to file claims that have a realistic chance to be granted in view of the best prior art known to us at the time of filing. 

Claim drafting is an iterative process. We start with a problem, list the features that are necessary to solve the problem and then we go back and see if the problem has to be change or if the solution has features that can be generalised without being too vague. In this respect claim drafting is an art. Claims that are too broad risk to fall accidently in prior art and have to be restricted. The restrictions however, have to be already built into the claim structure, as we cannot go beyond the disclosure of the original filing. We cannot restrict a claim by a technical feature that was not already described in the description. This explains why we have today so many subclaims, each subclaims being narrower than the preceding claims. One purpose of subclaims is to have build in fall back solutions if new prior art pops up during examination or later in an opposition.



The EPO usually requires the problem-solution approach to check if an invention is based on an inventive step. We cannot draft a claim that is clear without a clear idea what technical problem should be solved. The inventor knows at least the prior art to the extend of the product he wants to improve. In case of Nicolaus Otto we know that he was striving to improve the Lenoir engine. Closest prior art to him therefore is the Lenoir engine, regardless that closer prior art existed he was not aware of. For our simulation how a reasonable claim could have been formulated at the time under the requirements of today we therefore need do a problem and solution approach based on the Lenoir engine.  

We can find the problem to be solved in the original description:

  • avoid shocks by explosions


Let us assume that Otto was at least aware that Lenoir was using a diluted mixture of air and combustible gases to achieve a similar result.


The patent description discloses two embodiments:

Embodiment A is a gas engine without compression of the sucked in gases;

Embodiment B is a gas engine with compression of sucked in gases before the gases are ignited.

The description of embodiment A discloses that the cylinder on its way from a to b to suck in air and on its way from b to c to suck in an intimately mixture of air and combustible gases.

Therefore, sticking closely to the original claim, a claim that would cover the embodiment A could be formulated as:

  1. Method of releasing operational power by expanding gases in an enclosed space by combusting a mixture of air and combustible gases comprising the step of introducing into the enclosed space first air and subsequently the mixture of air and combustible gas.




These steps applied in that order moderate the combustion. Once started, the combustions propagates in a controlled manner. Instead of explosions with impulsive expansions the pressure of the expansions is evened over a longer time period. This reduces shock loads.

Now let's look if we can enlarge the scope of this claim. The original claim used the term "air of a different kind". Embodiment A does not give us any disclosure that would suggest that air of a different kind is something else than pure air.


From embodiment B we learn that the cylinder is enlarged at the end where the intakes for air and combustible gases are. Therefore the piston on its way from a to c three layers are created:

  1. mixture of air and combustion products of a previous combustion
  2. air
  3. mixture of air and combustible gases

 We could therefore formulate a dependent claim 2 in addition to claim 1:

  1. ...

  2. Method of releasing operation power according to claim 1 wherein combustion products from a previous combustion are introduce into the enclosed space prior to the introduction of air.


The description can then explain that the air and the combustion products, respectively, slow down the combustion as they envelop the mixture of air and combustion gas. Even if this explanation is wrong, it does not harm if it is in the description and not in the claim itself.