LTT24 - On the trail of the dropout on the engine test bench

Extremely short-term combustion processes at high speeds:

motor measurement technology has always placed special demands on data acquisition. Digital storage oscilloscopes offer the required sampling rates, but lack the familiar PC comfort. The PC alone, however, is overwhelmed by the flood of data. The intelligent front ends from LTT Labortechnik Tasler GmbH, which are optimized for high throughput rates, provide a remedy.

The Institut für Motorenbau Prof. Huber (IMH) works as a service provider and partner to the automotive industry and its suppliers in the field of engine development. Founded in 1948 by Dr.-Ing. Eugen Wilhelm Huber as a private research institution, the institute is now part of the MBtech Group and thus of the DaimlerChrysler Group. The current range of services extends from contract development and testing of engines and engine components, through the construction of prototypes and specific measuring instrument developments, to cooperative collaboration in the context of product development of systems and subsystems. For this purpose, various test benches are operated at the Munich and Stuttgart sites for the optimization and further development of diesel and gasoline engines.

Required: Sampling rates in the MHz range

A current task is the investigation of combustion optimization in gasoline engines, in particular the detection and analysis of misfires. Data acquisition on the running engine requires particularly fast and precise measurement technology with sampling rates in the MHz range. In addition to cylinder pressure, electrical variables such as ignition current and ignition voltage are recorded, as well as fuel pressure, for example. The cylinder pressure provides the trigger conditions - both for starting a power stroke and for detecting misfires.

For subsequent evaluation, the reference to the crankshaft angle with a resolution of 0.01° is also required. In the field of combustion chamber analysis, IMH has special expertise in the area of optical measurement technology (e.g. schlieren measurement technology) on transparent engines.

The schlieren method, which was developed around 1900, is based on the visualization of light refraction processes. These refraction processes are based on density differences, which in turn are caused by pressure or temperature gradients or by evaporation of fuel. In order to be able to image the liquid component in particular, as well as the fuel vapor and the spread of the flame core, a high-speed camera is used to record the working cycles at up to 40,000 images per second. In order to capture the decisive processes, the camera system must also be triggered by the measurement technology with microsecond precision. With the LTT24, the IMH has a measurement system that meets these high demands. This front-end system, developed and produced by LTT GmbH, the Würzburg-based specialist for ultrafast metrology, expands the bandwidth of conventional PC metrology into previously unattainable dimensions: Depending on the desired resolution, the possible sampling rate per channel ranges from 1 kHz to 2.5 MHz at 16 bits or up to 20 MHz at 12 bits. A single device offers up to 16 differential inputs. Since the devices can be cascaded, synchronized acquisition of several hundred parallel channels is also possible. Separate A/D converters and amplifiers for each input allow simultaneous sampling of all channels and channel-specific amplification with input ranges between ± 1 volt and ± 50 volts. Each input has an adaptive anti-aliasing filter.

Patented LTT measuring method

The fast data transfer to the PC takes place via a special, patented SCSI-IIInterface, optionally also via USB 2.0. The measuring device itself does not require any drivers in the PC, configures itself and is immediately ready for operation after switching on. The internal hard disk of the device is automatically recognized by Windows as its own system drive and therefore allows direct access from the PC to the stored measurement data by any application programs. The supplied software not only handles the entire hardware configuration, but also the parameterization of the measurement. This includes the selection of the recording mode (single or ring memory measurement) and the various digital and analog trigger options. Sampling can be equidistant on a time basis or externally clocked. This allows indexing of the measured values relative to the crank angle. Up to 16 such configurations can be stored in the device for measurements in stand-alone operation. The acquired measurement data is written either to a high-speed RAM of 128 megabytes (optionally up to 512 megabytes) or to the integrated, shock-resistant 60-gigabyte hard disk. With a PC connected, the signals can also be displayed and processed online. Further processing and evaluation of the recorded measurement data can be carried out in all common software formats (e.g. Diadem, µGraph etc.).