Why and how: the dip stick dilemmas

The handbook says check the oil with the car on the level. On holiday: everywhere nearby was sloping a little. Sure enough: at each place the level on the dipstick seemed different. His Irritated hunt for somewhere suitable gave The Lad ample time to reflect that the measurement of the amount of oil in an engine seemed remarkably low-tech in this day of high-tech engineering,. It’s like having to use your finger in the dark to find out how much beer is left in the glass.

What does the engineer demand that the oil do in a car internal combustion engine (ICE)? We will come back to that question.

Most other features of the ‘driving experience’ are either electronically governed or satellite mediated and unlikely to be less so any time soon. What is it about the oil? Even that most complex and technically advanced of power plants, the gas turbine aircraft engine, has a sight glass. That is, arguably, less advanced still than the dipstick.

By definition when the engine is running, oil is distributed throughout the engine and there is instrument power enough to measure pump pressure. But pressure does not vary with the amount in the engine. Until it is too late, that is. How are you to discover how close it is to running out? Firstly, the engine designer has to send the oil to one place to give us the chance to measure it. Clearly, it can only be allowed to go to that one place when it hasn’t got anything better to do elsewhere. Only when the engine is stopped can the oil go for its roll-call.

But when the engine is stopped, we cannot afford any significant, power drain from the battery; neither for pumping oil to a tank, nor for a powered measuring instrument. There is only one force to do the jobs. So, gravity it is.

The roll call takes place in one place: that place is in a container at the lowest point where gravity can be relied upon to drive the oil. To save space under the bonnet and give suitable ground clearance, this container will be relatively shallow, probably wider than it is deep. If you have ever tried to carry water in a shallow dish without spillage, you will know how difficult it is. The liquid is very eager to migrate to one end or to the other.

As an aside, it is worth showing how some things are important over a wide range of engineering affairs. In another part of the engineering forest, this liquid behaviour, called the ‘free surface effect’ is also very important to naval architects who are designers of ships. For them, in tanker ships and ferries the free surface effect is not just an inconvenience: its management is a matter of life and death. In 1987, 188 people died and many more were injured in the Herald of Free Enterprise when it capsized in less than 4 minutes after sea water entered an undivided, vehicle deck.

The engines that power most cars and trucks on the road are reciprocating ICE engines; that is those with cylinders with pistons in them. The Lad has never been involved in their design. However, as an engineer, it became clear to him that this eagerness to migrate to the ends of its container is central to the dipstick problem. It is this that means that the dipstick level is different when a car is on the level or on a slope.

Is there a position then that means the variation with any slope is small or, at least, as small as possible? Yes there is. The Lad set up a toy demonstration of this effect. Below is shown a container with three [not very clear] red marks at the liquid surface. First, the surrogate tank is level and each red mark is at the liquid level.

Level tank overall
Model Oil Tank on the level

 The views expressed in this post are not necessarily those of Kempe’s Engineers Year-Book. No engineering book was harmed in this demonstration.

Now if we tilt the ‘tank’ down at the left as if the ‘car’ was on a slope, what do we see? The liquid level rises above its left hand mark and sinks below its right hand mark. Not surprising.

Down at the left

The next image shows the effect more clearly.

Down at the left enlarged

Now, tilting the ‘tank’ down to the right; the level on the left falls below its mark.

Down to the Right.

The point that we are making here is that levels measured close to either end of a tilted tank vary significantly. These images show [although far from Wallace and Gromit standards] within their limits, one important effect. There is one place to measure where the tilt has no effect. It is half way between the tank ends.

So: we see that breaking the liquid surface midway between the oil sump sides is the best place to site the dip stick. Then any slope will have no effect on the measurement.

Clever, eh? So, there we have it. The student interview syndicate answer! Right!


Here is the dilemma. How do we make it accurate in a real engine? The dipstick cannot go anywhere you like. It has to take account of the engine structure for one thing. The pistons and cylinders and valve camshafts tend to get in the way; not forgetting all the rest of the components, large and small, that pack the engine compartment these days.

Now The Lad is not an ICE specialist and the above is naive perhaps or too simple. So who would know?

The UK company, Ricardo is one of the foremost specialist ICE designers. They have been engine designers for nearly 100 years, working on a very large number of projects, Their market has included defence, motor sport and marine. They worked as part of teams with General Motors and Chrysler of the US to design a class-leading V6 engine capable of global implementation for GM and, for Chrysler, the Dodge Viper engine upgraded to 8.4 litres and a massive 600 horsepower for the ultimate muscular American sports car.

Ricardo engineers work on driveline and transmission system engineering. There have been cost-optimized manual transmissions for developing markets and advanced and high performance systems such as the dual clutch transmission of the Bugatti Veyron.

In short, they are proper engine designers; not ICE dilettantes like The Lad.

A Ricardo engineer put it this way. The dip-stick provides a rough estimation of oil levels in the sump but it’s not necessary that this is wildly accurate. We can tolerate the effects of slight changes in pitch/roll of the engine in comparison to the acceptable level between maximum and minimum recommended fill which is very large. In practice the dip stick is usually placed mid-way along the engine but again, this isn’t particularly critical provided that oil level is being checked on reasonably level ground (as specified by almost all manufacturers).

During the engine design, as the Ricardo engineer put it, the design engineer knows that it is essential that he or she ensures that there is enough oil capacity and enough oil circulating to maintain sufficient cooling to avoid overheating the bearings. These days, oil coolers are also commonly used for this purpose, especially in hot climates or where the vehicle is working under extreme loads (e.g. towing in mountainous areas).

During engine operation, the crucial consideration is that there is oil at an acceptable temperature available at the pump intake when the engine is operating – this will typically be drawn from around the lowest point on the circuit. On the other hand significant over-filling should be avoided as this only causes wasteful churning and possibly some oil reaching the combustion chamber. The quantity of oil specified for a given engine is not an exact science but a compromise based on the above comments – usually erring on the conservative side.

There, now, you really do have it!

The engineer’s emphasis was that these are the key points in the mind of the ICE designer; not agonising over the height to a mm of the oil surface. Good ideas, even clever ideas, are not enough. Engineering for the designer is to have, backing her up, real experience of the components in the gritty world. The engineer is always pragmatic at heart and the real-life solutions are usually complicated and not always tidy. Engineering teams, like Ricardo and others like them at the top of their game, have this experience. If you want to be an engineer, seek them out.

If you are really in difficulty finding a level surface, there is an approximate solution. Check the dip stick with the car facing one sloping way, then turn the car till it is facing in precisely the opposite direction and note the dip stick again. Half way between the highest and the lowest is a good reading.

You would be right but only partly so if you said the job of the oil is to lubricate bearing surfaces. An equally important task is the cooling of the core components of the engine that external coolant flow cannot reach.

Engineering is one of the three drivers advancing the human race. This blog describes real professional engineering as it is in the real world. It is not well served by the current media. An engineer is posting: not a ‘scientist’. The Lad is entirely independent of any organisation mentioned. The target of the blog is the career seeker and the general public.

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