Short survey of air filters

It is a familiar sight for German armour enthusiasts: one or two white boxes dotted around a Panther (or Tiger II) wreck. Usually these boxes seem to have been thrown down carelessly and are not in the best of shape. I began to wonder what the function of these boxes was and why they were thrown down there just like that. In this article we are going to talk some more about these “boxes”, which are in fact the air filters of the Panther’s engine, the Maybach HL 230 P 30.

First of all I would like to mention two nice examples of Panther wrecks where the air filters are clearly visible. As you can see, the filters can be found in anything but their usual place – on top of the engine.

From textile subcontractor…

Our story begins in Ludwigsburg, Germany. It is January 1941 and the company Filterwerk Mann & Hummel GmbH is founded by Adolf Mann and Erich Hummel. Both men are entrepreneurs and managers of the Bleyle company. As a subcontractor, Bleyle did not succeed in obtaining the “Kriegswichtig” seal as indication that their work is important for the war effort: something that Mann and Hummel would have liked to see happen in order to guarantee the continued existence of their company. As a subcontractor, Bleyle has good contacts with the company Mahle KG, which manufactures, among other things, aircraft parts, engines and air filters. In contrast to Bleyle, the production of Mahle is considered to be crucial for Germany’s war machine. Because of this, Mahle is overloaded with orders from both the Wehrmacht and Luftwaffe. Since the production capacity at Mahle is fully utilized, Bleyle is approached to relieve the production. ​[1]​

…to filter specialist

When Mahle KG decided to completely outsource their outdated filter production lines, Bleyle stepped in and it was agreed that they would start production of filters under a license agreement . For this purpose Mann and Hummel founded the company Filterwerk Mann & Hummel GmbH. They leased machines and workforce from Mahle, but moved to their own location. From June 1942 the department was completely owned by Mann & Hummel when the lease with Mahle was dissolved and the department was finally bought. It was also agreed that Mahle would not be active in the filter branch for the time being. The company decided to focus mainly on filters for armoured vehicles and airplanes: each indispensable for the war. In 1942 it was decided to focus on innovation and the R&D department was expanded considerably. ​[1]​

Importance of an air filter for internal combustion engines

It goes without saying that combustion engines need air to burn fuel. The controlled ignition in each cylinder leads to movement of the crankshaft. The rotational power can then be used to drive wheels or the tracks of a tank. However, there are always dust particles in the air. When these dust particles enter the cylinder, they rub against the cylinder walls as well as the piston rings. Wear and tear of the cylinder tube and piston rings lead to leaks and reduced compression which result in increased fuel and oil consumption.

Technology

Before we continue we have to make a short trip to the air filter technology. I will only discuss the techniques that are relevant to us in order to keep the article somewhat short and concise. 😉

Wet air filters

In the beginning of the 20th century wet air filters were used to purify engine air. In this type of filter, air is sucked in through a filter element consisting of steel wool or a fibrous material soaked in a liquid. In the beginning this liquid was often just water, but later on more and more often oil was used. While the dust particles collide with the many contact surfaces of the filter element where they stick to the oil, air makes its way around these obstacles without too much effort. This type of filter is very maintenance-intensive, because it only retains its properties as long as the filter element is wet and the oil is not saturated with dust. The filter element can be regenerated: after rinsing it out with fuel and moistening it with new oil, it can be reused. ​[1], [2]​

Oil bath filters (Wirbelöl- or Ölbadluftfilter)

This further development of the wet air filter is provided with an extra step in the form of an ‘oil bath’. The dusty airflow is led over the oil bath in a sharp bend. Due to the inertia of the dust particles, they will become trapped in the oil. The clean(er) air goes on to the filter element. An additional effect is that the air causes the oil to swirl and splash against the filter element, as a result of which the filter element partly cleans itself and requires less maintenance ​[3], [5]​.

Cyclone separators (Zyklon luftfilter)

An alternative method of removing dust from air (or other gases) is to use a cyclone separator. Air is sent through a special contraption to induce a spiral vortex – like a tornado. Like in a centrifuge the dust particles are thrown outwards when entering the vortex. Due to their inertia, the dust cannot make the tight curve required to exit the contraption and instead ends up in an outer downward vortex to the bottom, from where it can be extracted separately. ​[6]​

Because cyclone separators have no moving parts or filter inserts, they are low-maintenance: all that needs to be done is to remove the accumulated dust. A disadvantage of a cyclone filter is that there is a pressure loss; the outgoing air pressure is lower as a result of the cyclone effect. This prevents 100% efficiency of these filters in practice. In general, cyclone separators are therefore not used on their own, but as a pre-filter linked with other air filters. ​[2], [6]​

The advance of the pre-filter

The first patent for a cyclone separator was already filed in 1886 by Morse ​[6]​. Initially, centrifugal separators were mainly used industrially. After the invention of the car this principle was soon adopted by the German automotive industry in the 1920s ​[7]​. One of the pioneers in the field of cyclone dust collection was Professor Eugen Feifel. In the 1930s, Feifel published two papers, “Zyklonenentstaubung” and “Zyklonentstaubung, die ideale Wirbellsenke und ihre Näherung“, in which he describes formulas for the ideal cyclone for de-dusting applications.

Due to the increasing demand for high performance air filters in increasingly dusty environments – such as North Africa – for ever heavier engines, Mann & Hummel begins an experimental design for cyclone pre-filters on the initiative of the Wehrmacht ​[8]​. Based on the efficient formulas of Professor Feifel, a pre-filter is produced to spare the oil bath filters (which are also made by the same company) ​[7], [9]​. We now probably know these filters as the “Feifel” filters on the Tiger tank which were mounted on both sides of the back of the tank. An in-depth look at these filters can be found on Tiger1.info. I don’t know if these filters were known at the time under the name Feifel. In the Tigerfibel (D 656/27) they seem to be simply called Vorfilter (pre-cleaner).

The filters designed for the Tiger tanks appear in 1942, but are not fitted as standard. Eventually these filters were discontinued in October 1943 ​[10]​. A permanent solution was found in the form of a compact combination of air filters also from the firm Mann & Hummel.

Enter the Kombinations-Luftfilter

The Kombinations-Luftfilter LZ 224-mld is a multistage filter consisting of a cyclonic pre-filter and an oil bath filter. The filters are contained in a box-shaped filter unit with roughly the following dimensions (in millimetres): 666 x 426 x 219¹. These filters were specifically designed for use on Maybach’s Hoch Leistung 230 engines². In practice, these filters were only used with the P 30 variant and not the P 45 as the Tiger I did not have the required piping in place ​[11]​. Surprisingly, an (undated) Maybach manual for the HL 210 and HL 230 makes no mention of the combination air filters for the HL 230 P 30 at all: the manual assumes the installation of two oil bath filters ​[3]​.

The filter unit consists of a centrally located drum-shaped oil bath filter. On both sides a pre-cleaner in the form of a “cyclone battery” is present, consisting of 12 parallel cyclone separators each ​[5], [12]​. Every separator has a slot on the bottom that allows dust to escape, while clean air continues to the next stage. The oil bath filter itself consists of three main parts: an oil pan, a lid and a filter element which is located in the lid.

Dust particles leaving the separator enter a chute. The chutes end up in one of two Staubkaste or ‘dust boxes’. The dust boxes are not a part of the filter units, but are a mounted to the valve covers. To avoid the need for maintenance, the dust boxes are connected to the fan in the adjacent bay such that any dust is directly extracted. The triangular shape and sloped design aid dust extraction. The photos below show the location of the dust boxes. The rectangular slots are where the chutes of the filters would rest.

It may be interesting to note that the filter unit is not symmetrical. The position of the unit is offset in relation to the carburettors, therefore the chute on one side has to span a larger distance to the dust box and hence is wider than the other.

Maintenance

The D-vorschriften ​[5], [12]​ advise to clean the oil filter every 250 km whereas the cyclone pre-cleaner needs to be cleaned every 1000 km. Below is an image of a filter unit (originally from Saumur’s Königstiger) which still has the original label with maintenance instructions on the side. The text is hardly legible, but the original text is known:

Kombinations-Luftfilter LZ 224-mld
1.Inbetriebnahme:
Deckel abnehmen. Patrone herausnehmen.
Filtertopf bis zur Ölstandmarke mit Öl füllen.
2. Wartung und Reinigung:
Filtertopf und Patrone in Benzine oder Rohöl aus-
waschen. Neues Öl bis zur Ölstandmarke einfüllen.
3. Zur besonderen Beachtung:
Dichtungen zwischen Luftfilter und Vergaser, an-
beiden Staubabsaugstutzen, sowie am Deckel von
Schmutz freihalten und auf richtigen Sitz achten.

The photos below show the process of removing the lid on the oil pan such that it can be refilled with oil.

As part of the restoration of several Panthers for the Wheatcroft collection, a number of air filters have been reconstructed by Ben and Jordan³. In five videos Ben Jackson clearly shows the working principles and different parts.

Footnotes

1. Approximate dimensions extracted from Maybach HL 230 engine drawings on archive at DGA and NARA[↩]
2. The same filters appear on a drawing for Maybach’s HL 234 engine[↩]
3. posted by The Wheatcroft Collection on 4 March 2019[↩]